The rumor of a "one and done" Mac Studio were from a Gurman report, so take that as you will...
I think Gurman has a bobby horse of trying to bring the large screen iMac back. so killing off the Studio makes hjis "new iMacs coming" theory ring more 'true' .
It can't possibly be the "Mac Pro" and the Mac Studio have high overlap with if the Mac Pro starts at $5K (or somewhat likely $6+ K ). Apple themselves stated that the Studio was a replacement for the iMac 27". if the new Mac Pro blows away both the 27" iMac and the current Mac Pro 2019 space that would be a huge 'ask' for just one product. The Studio starts at $1,999 ; that is a $3+ K gap between that and likely Mac Pro starting point. More than big enough to segment the products.
[ There are some overlap areas. if the Studio and new MP both have a Ultra then if configure up the Studio to full Ultra and 4TB drive get close to the MP starting point. But if start the Mac Pro with full Ultra and a base of 2-4TB of SSD .. can raise the MP 'floor' so not as much of an overlap. And if selling an Ultra SoC either way ... not much different than an M1 weather MBA , MBP 13" , 24" iMac situation. ]
Tossing a Mn Pro into a Mini isn't going to cover the space the Studio Ultra is in either.
If Apple dumps the Mac Studio it would more likely be to bring back the large screen IMac. Not that Apple dragged the Mac Pro back down to a $1,999-2,399 price point again.
There will be some substantive product differentiation issues if the Mac Pro is the "one slot wonder" that has been rumored at bit. If it only had some relatively crippled slot ( e.g., x4 PCI-e v4 worth of bandwidth ), then there would not be much of a gap between it and the Mac Studio when it came to I/O. But if it has two (or three) x16 PCI-e v4 full length , full height slots ( 6 pin aux power connector(s) ) , internal SATA and USB dongle connectors (like MP 2019 ) and fit the J2i drive sleds (and the similar brackets ) there would be a large 'value' gap between a "Ultra Studio" and an "Ultra Mac Pro". And they wouldn't be in the same price range. [ Likewise if have a 'rack' Mac Pro version would 'play' in vastly different space than a Studio chassis. And even bigger gap if Apple provisioned capabiltiy of putting a "compute" GPU (e.g., AMD MI210) in there. ]
Gurman's older Mac Pro rumors were about a "half sized" Mac Pro. If that is "half the volume" ( as opposed to something constrained to Apple's literal desktop footprint constraints of a 8"x8" square) then Mac Pro would be substantive bigger (and extremely likely more expensive) than the Studio.
If Apple were to 'shrink' the Mac Pro to just slightly bigger than than a Studio ( e.g. one 1/2-3/4 length slot and half height slot ) then it really would not be the "Mac Pro " pushing the Mac Studio out of the line up. It would more so be Apple 'upgrading' the Studio system with the old "Mac Pro" name ( pragmatically dropping the legacy notion of a Mac Pro altogether. MP 2013 design mantra is the new 'king' . ). Putting the "Mac Pro" name on a 'taller Mini" chassis probably isn't going to work long term with the current Mac Pro user base. The likely only way the Mac Pro 'replaces' the Studio is if Apple has designed a 'bad' Mac Pro. Perhaps, he is covertly trying to say the implementation is not really a Mac Pro.
Really would have to see a market breakdown of just how many Studio Max versus Studio 'full' Ultras Apple is selling or not. If the vast bulk are Max and binned Ultra SoCs then really not much of a 'problem'. Gurman is inventing a 'problem' that isn't there.