But they need to upgrade the abortion that is the Mac Pro right now.
There is a big gap between "they need to" and "they might like to" . Apple doesn't "need to". Folks posted 100's of messages to that same affect during the 2015-2018 time period and Apple delivered nothing. Primarily because they didn't "need to" ( it wasn't going to significantly impact the tactical or strategic direction of the Mac ecosystem at all. Apple grew revenues and unit volume during that period just fine. Hence no 'need' fiscal , strategic , or tactical 'need'. )
The M3 gen probably isn't going to 'cure' soldered RAM , iGPU only , and BTO pricing for upgrades. Very likely a large block of MP 2019 users will grumble at the M3 version also and simply buy used and/or 'bargain bin' old RAM , video cards to keep their current, Intel systems going for another couple of years. ( very much like what happened in 2015-2017 time frame when the MP 2013 was the 'abortion'. ). As long as macOS on Intel is getting active , robust support many of them are not going to move.
An M3 Ultra would actually be very competitive across the board except for the scant few die hard 3D GPU rendering guys who want better than RTX 4090 (A6000) performance.
For the primarily 'hyper focused' GPU computational rendering folks, there isn't going to be a large gap between the MP and Mac Studio(MS) if there is MP is still only stuck with just one, shared with MS, SoC then there is really no hurry. If the GPU is the same and the GPU is the just about the only thing that matters, putting the M3 Ultra in a MP isn't really going to sell to that particular crowd. Bulk of them with now substantive additional I/O throughput and/or data storage capacity needs are going to take the relatively more affordable option.
Folks who have a higher priority focus on non GPU I/O throughput are still going to perfer the M2 Ultra MP to the M3 Ultra MS ( lack of access to the large PCI-e backhaul bandwidth would still beat the TB4 sockets on a M3 Ultra system. )
"But this will mean there are more Studio sales than MP sales ". That is already true. M3-gen very likely isn't going to change that. A mystical M3 Extreme very probably would not either if it is priced extremely high.
Furthermore if Apple skips the M3-gen then the MP could relatively easily jump onto the M4 generation sooner rather than later. If Apple 'saddles' the MP with paying off the M3-gen then it cannot jump to the M4 generation sooner ( it has to help pay off the M3 work. ). The MP coasting on M2 Ultra would extend the lifecycle of both the support to that SoC ( Apple's vintage/obsolete countdown clock starts when the stop selling something. Discontinuing the MP 2023 faster means it is desupported faster. After spending $7-10+ K a substantive fraction of the owner userbase aren't going to prefer than option. Higher prices typically lead to systems being pressed into longer service lifetime cycles. )
If Apple had waited another 3-4 months to release the MP 2023 they could have placed a M3 Ultra in it in the first place. Pragmatically they could delay longer because they needed to complete the transition and MP 2019 was so 'old'. That baseline of the MP refresh cycle be long very likely has not gone away.
The initial release was coupled to the Studio's M2 transition , but that very likely is not necessary ( a requirement level 'need') over the long term. [ The Mini Pro didn't have to ship with the MBP 14" Pro, the iPad Pro didn't have to ship with the iMac M3, etc. etc. ]
The 4090 is on track to be superseded in late 2024 - early 2025 anyway. If Apple is going to try to 'chase' those class of cards they would be a system priced more like the studio to do the chasing , than one with higher upfront costs like the MP. Nvidia has already release a subset of the 4000-super offerings already. If M3 ultra is Q2-Q3 2024 Nvidia is likely to have moved the line anyway. There is always going to be a substantive gap between Apple and 2-3 other competitors who can release at different times spread out over the calendar year. In this space, Apple should deliver when they have very good , solid updates , than fixate on somebody else's calendar. ( AMD or Intel shoveling stuff out the door to beat Nvidia to some initial demo day fails at least as often as it suceeds. )
I am pretty convinced that the M3 Ultra chip will pretty much be faster at everything compared to any 2019 Mac Pro config that is running OS X.
A likely significantly enough large fraction of configurations. However, folks with a 400-600GB (ECC) RAM data set requirements ... probably not. Folks with > 128GB VRAM resident problems probably yes.
Apple doesn't need something that covers every possible slot configuration of the MP 2019. They just need to cover enough so that they can get a decent ROI on doing a new MP. That is what is questionable. Just throwing 'specs' at the folks who didn't move with the MP 2023 isn't going to move most of them. It is 'form' (hyper modularity) not 'function' that a large fraction of the non-movers are hesitant about.
Of course the case for keeping on Intel is so that there is a viable dual boot windows mechanism. 🤷♂️
Apple should just put the M-series on a standard PCI-e card ( maybe squeeze M3 Pro onto a bus only powered card. Or bite-the-Aux-Power-bullet and do a M3 Max+ card. ). That way folks on legacy MP 2019 , Windows (or Linux) , and MP 2023 folks could throw a newer Mac into their systems. They wouldn't have to 'dual boot' again. They could have them both booted at the same time. Run a virtual Ethernet via the shared PCI-e bus so can mount shared disk and/or 'remote' screen on a private network.
If could put a "Mac inside Mac" that would be a bigger functionality gap between MP and MS. One of the current MP problems is Apple isn't doing much effective on differentiation. There is a small subgroup they are targeting that doesn't to be expanded to everything for everybody, but it could be bigger. Especially if willing to just run concurrently with something else ( like they do in many more places now because cannot dual boot).