The MPX was an expensive adventure for customers or Apple? For Apple, the MPX connector ( in an of itself) probably was not relatively expensive at all (**) . Apple had jump some hoops to route mulitple Display Port streams through a switch and hither-and-yon all over the logic board to remote ports. But that became a solution in search of a problem when just move the Thunderbolt controllers onto the main die. Even Intel moved TB controllers onto the main die in the laptop models in 2020 before Apple even launched M1.
The R4i is a rather poor justification for the MPX connector. It doesn't use 3/4 of the connector pins present. It also likely doesn't use anywhere remotely near 150W let alone the full budget of the connector power. Unless had some older , power hungry 3.5 HDDs , 75W is a decent budget to run four drives and a small embedded controller SoC with some RAM. 75W comes off the basic bus for PCI-e. And the R4i soaking up the x16 PCI-e v3 bandwidth for just 4 HDD disk drives ... another
gross waste of potential resources.
The J2i product worked fine with a couple of cables. It also was 'cloned' as a product by multiple folks. For example,
3-drive mounting system for Mac Pro (2019). Install SATA 3.5-inch hard drives and 2.5-inch SSDs. Kit includes brackets, screws, and cables.
www.sonnettech.com
[ Folks who just wanted an internal "Time Machine Drive" ... the J2i worked fine. Apple made "internal TM" popular on previous Mac Pro's so there was demand that pulled in solutions from multiple vendors. ]
The R4i was cloned ( best I can find) by
NOBODY. Relatively extremely straight forward engineering changes could have turned it into a 6-pin + bus , or maybe bus only, powered card with no real material differences in feature set. The R4i 'smells' a lot like the LG UltraFine 21.5"/27" products. Apple kind-of-sort-want wanted something but didn't want to make it themselves. So they go to a vendor and to some "back seat driving" of the specs. ( "make a monitor with one and only one input, no buttons , etc. ... Yeah nothing like the other stuff you sell." "Make a huge card that takes four 3.5 drives with now wires for power. Yeah, nothing like the other stuff you sell at all") . In exchange for the pushing the constraints on the design, Apple promises to buy a sizable block of product ( should help mostly/partial pay for R&D indirectly ) and vendor gets "most favored " status on the Apple Store. Lots of exposure in tech docs and sales material and Apple Salesfolks training sessions.
[ Note: nobody every cloned the LG UltraFines either with Apple's imposed design constraints. ]
The R4i was deeply born out of the Power Mac G5 and the carry over of the 4 drive bays (and a bit of Apple "real" RAID card). Apple felts they couldn't dislodge some folks from their 2008-2012 models unless the customers could pull those 4 HDDs in the trays and move them over.
The Pegasus M4 ( now R4 ) works across the whole Mac line up and isn't materially slower at all. Hence the R4i is 'covered' in at least two directions by just Promise products. J2i is way more affordable. M4 is way more flexible.
Throw in modern PCI-e cards with multiple U.2 or M.2 SSDs. deliberate large SSD cards. Upcoming E1/E3 EDSFF drives. etc.
** ASUS whipped together their own power only , "look ma no wires" connector for GPUs.
Replacing the 16-pin power connector with a proprietary slot.
www.tomshardware.com
(possibly after the 16-pin drama broke out). Probably didn't have a ginormous R&D budget to do that.