Right now, what would a Mac Pro offer that is markedly different then the studio?
Flexility in expansion.
I have Mac Studio and I can passionately argue how great it is for my needs. However, it's basically a completely, locked-down Mac mini... a fat Mini if you will.
Mac mini has filled the same basic role of self-contained, headless Mac at the low (price) end for a long time. At the high end, Mac Pro has been the other version of a self-contained, headless Mac. Mac Studio plops in somewhere in the middle though it is more like the Mini than the Pro. Why? What has Mac Pro offered vs. Mac Mini all along?
Slots. "We" will marginalize slots like crazy now ("99% don't need...") to help push Studio to those worried a new Mac Pro won't have slots, but slots open up many niche uses for a Mac that are not fulfilled by what Apple decided to jam into a single, locked down SOC. Anyone who needs those things need a slot(s) into which to put that extension hardware. No Mac with slots- no way to use what they need to do whatever they do with Mac Pros. Hello PCs with slots if Apple doesn't deliver a Mac with them... not because people want to defect to Windows/UNIX but because Apple has no Mac to give them what they need.
Big RAM. Some need RAM beyond the upper end of what Apple chooses to supply on the SOC. Mac Pro has tended to offer substantial RAM expandability and slots offer even more of such opportunity. Those who need big RAM can't magically hop on Studio Ultra and everything that demands the huge RAM just happily adapts to the MAX config. RAM is key to fastest processing and more RAM typically means faster processing. For many computer buyers, SPEED to completion of computer tasks trumps what "we" spin to argue against anyone needing more RAM than what Apple chooses as max config options now.
Evolution. Traditionally, the best graphics card at any given time is supplanted by better ones within a year or two at most. Mac Pro can "keep up" vs. "throwing baby out with the bathwater" because the graphics capabilities get too far behind the times. Similarly, some people want a computer that can evolve with their unanticipated future needs vs. trying to guess exactly what they will ever need and buy it up front. Mac Pro is the ONLY Mac that can evolve inside the case to keep up with advancing, supporting technologies.
Horsepower. Mac Pro has traditionally been the most powerful Mac... not the most power efficient Mac. Power Per Watt is not generally a consideration with this Mac. It is supposed to be the most powerful Mac. Those who pay the most for a Mac are generally wanting a Mac that can compute FASTER than any other Mac and, ideally, any other PC. "Our" push for PPW so that Apple can win a head-to-head contest vs. PC comparisons misses the point here. Computing speed translates into getting tasks done
FASTER. If that takes a little more electricity to get things done faster, this kind of buyer is good with that.
My own Mac Studio Ultra is great and all but there are no slots for specialized hardware, RAM is locked at the choice I made at purchase no matter how much I might ever need, the only possible Evolution will be in software (it will NEVER have a better graphics "card," nor adapt to unanticipated future needs unless whatever that might be can be external via thunderbolt/USB (which is NOT a lock at all)). MAX horsepower is
already inside; it has no path to become more (hardware) powerful at any point in the future. Objective reviews show that higher end, newer Intel hardware is already more powerful (yes, at the expense of using slightly more electricity)... but, bottom line, more computing horsepower gets more done FASTER and sometimes getting work done faster is much more important than shaving a dollar or three each month off of the electric bill.
My first Mac was "pro"-like: the PowerMac G4 "tower." During it's useful life, I did eventually encounter use situations where the card slots came in very handy. I didn't anticipate those needs up front but it was nice to get that added value of it when such needs arose. Computer owner situations change/evolve- sometimes in unanticipated ways- and it's nice to have a computer that can evolve with whatever happens. As is, ALL Silicon Macs are completely locked down... basically peaked in their capabilities for their buyers on delivery, unless some want can be covered in software updates only. Even my Mac Studio Ultra is all it will ever be on the hardware front. Should I ever need a computing something that can't be addressed in software only, I have to buy a whole new computer.
That last line is why there has long been a group want for the so-called x-Mac... a Mac between Mini and Pro with a few slots... the equivalent to a PC mini-tower if you will. Instead, Mac Studio is slotted in there now and it is locked down. Want/Need to update the graphics? Nope. Want/Need to add some additional RAM? Nope. Have some niche hardware need on an industry-standard card? No card slots for that inside or outside of Mac Studio. Etc.
I hope this helps.