Apple, like other OEMs, gets discounts from Intel for shipping only Intel processors in their computers. Presumably these discounts are ending with the upcoming Arm switch, but up until now Apple has continued to receive them including for the Mac Pro.
Getting discounts for bulk buys is significantly different than a lock out contract that bars Apple from buying from another vendor. Apple makes lots of major investments not to end up in single vendor lock-in situations long term. As for as "needing" the Intel discounts, Apple sells Macs CPUs for more than normal ark.intel.com 1,000 tray prices. Mac doesn't have "Intel inside" stickers on them for Intel kick-back discounts either. Apple isn't financially "pressed" to need those kick backs to have a profitable system business.
If was/is financially more convenient for Apple to max out their bulk discounts with Intel while AMD didn't have products that were an equally high performant and Apple objective aligned products. AMD has been cheaper for a long while. If Apple was primarily concerned about cheaper they could have moved to AMD a long time ago. AMD being somewhat hostile to Thunderbolt was probably a bigger issue than cost. Intel willing to co-work on projects that Apple cared a lot about mattered. AMD has come around on competitive CPU product delivery and less hostile against Thunderbolt ( now that basically part of USB 4), but they haven't dug themselves out of the hole faster than Intel dug themselves into another one that Apple doesn't like. AMD is also primiarily focused on desktop where Apple sell 75+% laptops.... so unless myopically looking into the iMac 27" BTO , iMac Pro, and/or Mac Pro space.
Apple also generally avoids selling "loss leader" products where many system vendors need Intel's discounts and kick backs to make them closer to break even ( less underwater ).
Apple’s plan was to replace the trashcan Mac Pro with iMac Pro, but in April 2017 they showed off their yet-to-be-released iMac Pro to some key pro customers. The customers were thoroughly unimpressed and pointed to their cheesegrater Mac Pros. Having already committed to iMac Pro Apple quickly threw together a round table press event and announced the upcoming modular Mac Pro in order to prevent their pro customers from abandoning them when they released the underwhelming iMac Pro. Apple then threw together a team, told Johnny Ive to build them a cheesegrater, and 2 years later we got the current Mac Pro.
Highly likely that isn't completely true. The vast majority of the 'die hard' customers that were pointing at their Mac Pro 2009-2012 models had been pointing at it since 2014. It isn't like most of them they had some sudden preference change in 2017 that Apple had never heard of before. Apple new the Mac Pro 2013 had issues in 2014-2015. They largely went about 'solving' them with the iMac Pro as opposed to completely back tracking. But it is highly doubtful that Apple was not getting feedback in that time period from a substantive subset of users that another "box with slots" was also a preferred option. [ For example, there were multiple thread that each spanned , muliplte years years (and thousands of posts) complaining about the Mac Pro 2013. Mulitple times in each thread where folks suggest sending grumblings off to apple feedback. Mulitple sites where the same thing. Additionally, there were multiple threads on multiple sites complaining about failed GPUs . Similarly to the butterfly keyboard thing... it wasn't that Apple didn't 'hear' earlier that they had a problem. There is difference between when they 'hear" and when willing to walk back some design decision that has a large chunk of ego attached it. (e..g, "Can't innovate my a** " , "Courage to kill the audio jack". Apple is in now way oblivious to the range of feedback about those moves. )
One primary reason for the April 2017 "dog ate my homework" meeting was that the 2013 Mac Pro was about to hit the
1100 days mark in age. It was going to be a wide tech press "moan and ground" festival of complaining about how comatose the Mac Pro was a product. It was far more a bad press management event than a "sudden discovery" issue. In April 2017 Apple tweaked the Mac Pro default configurations and sites like Macrumors mercifully reset the "age" date on the system (temporialy). Apple had set a signification 'record' for inaction. Every addition 100 days was a new "even number" record to grumble about. They needed to make some overt excuses. Apple knew they hadn't put in work here. That they suddenly discovered no work is just probalby just excuse making for them on the outside.
( the Mac Mini was also well on track to be yet another desktop product setting records for delay at that point also. It wasn't that the Mac Pro was the only comatose product in the line up. )
Similarly the Mac Pro 2013 didn't try to cover both halves of the Mac Pro 2009-2012 design that. There was a "go large" element to the dual processor system. In terms of RAM capacity and an even higher core count. ( MP 2013 covered the MP 2012 max 12 core count. It didn't really move it up. ).
In one of the interviews with Apple folks after the Mac Pro was released it got revealed that the iMac Pro and Mac Pro have the same product manager. That's is pretty indicative that these products are pragmatically being single tracked. It probably wasn't that Apple had to "scramble" to get the Mac Pro started in 2017. It is at least just as likely that it was the Mac Pro's 'turn' to get started in 2017-2018 after iMac Pro got primary focus in 2015-2016.
Previewing the iMac Pro to those customers was probably more so about identifying "what's left uncovered" than some kind of scrambling . Apple probably had not decided exactly how far more upscale bandwidth wise they were going, but the notion that they were completely blind that there wasn't more is exceedingly dubious. Xeon W in the iMac Pro was leaving loads of bandwidth on the floor. One x16 PCI-e bundle to the GPU , two x4 bundles to Thunderbolt controllers , and on serious overkill x4 link to 1-10GbE controller left 20 PCI-e v3 lanes completely unloaded. The iMac Pro was leaving performance on the "floor" just to fit inside the general 27" iMac enclosure constraints. The fact the iMac Pro was constrained to use only a lightly modified iMac enclosure is also indicative of how little independent , concurrent product development is done in the Mac division.
Apple had time to kick the can on a Mac Pro 2012 replacement because it had a 7 year Vintage/Obsolete count down that started in 2013. The folks who squatted on the model the base 2009 infrastructure for 4 years from 2013 to 2017 could probably squat longer. So the Mac Pro was ordered after the iMac Pro in priority. Especially if Apple enabled some new GPU cards to be added to the system with supported drivers for 3rd party cards. ( which the eGPU effort needed anyway).
I wouldn’t expect Apple to release a Mac Pro update until at least 2022, by then we will know more about what to expect from Apple’s ARM Mac processors.
I don't expect much new clarity before WWDC gives some public insight into macOS 11.1 . If the macOS 11.1 on Apple Silicon version still has "zeroed out" 3rd party GPU support then a new Mac Pro probably is deeper into 2022 than sooner. If the first Apple Silicon Mac lacks a Thunderbolt port ... similarly the Mac Pro is probably further out. ( as there is no easier access "developer and/or tester" test harness to work on enabling that 3rd party GPU support. )
Not only new SoC has to arrive to enable the Mac Pro , Apple is also doing a major disruption of the software drivers for macOS at this point also. The driver disruption impacts the x86-64 Macs also, but there is going to be lots more of those that need fixing for more than a year or so of rolled out Apple Silicon system sales. There are multiple holes to plug in multiple dikes.