Although I agree on your vision about former 27”iMac Pro, I feel it may be too optimistic to expect similar price to MBP. Considering Display size alone (regardless if Apple updates both DPI or “just”refresh rates), probably will make it at least $1k more expensive. So a 27” iMac (Pro) M1Max 32RAM 1TB around $4.5k? That could be “matched” by similar specs Mini “Pro”+27” Display combo at $4.5-5.0k?
I agree with this analysis and I think it's instructive to walk through it in detail.
The 14" MacBook Pro starts at $1999 with a binned 8/14 M1Pro processor, 16GB of RAM, and a 512 GB SSD. But a 24" iMac with an 8/8 M1, 16 GB of RAM, and a 512 GB SSD sells for $1899. If Apple were to offer the same quality screen as the 24" iMac in the 27" iMac, with a vanilla M1 and those specs, it might sell for $1999, but even that would seems to be a slight bargain relative to its smaller-screened sibling. If Apple wanted to make that machine (and, admittedly, there is probably significant demand for such a thing), they would have done so long ago. I would be very surprised if the 27" iMac (whether it's labeled "Pro" or not) did not have, at a minimum, an M1Pro and a screen of the quality of the 14" MacBook Pro. To anticipate the cost of these upgrades, I think it's best to compare what those upgrades cost with the laptops and extrapolate from that.
The 13" MacBook Air, with 16 GB of RAM, a 512 GB SSD, and a non-binned M1 chip costs $1449. I think the MBA is a better parallel to the 24" iMac than the 13" MBP because of the thinness of design for both machines, but for what it's worth, a 13" MBP with those specs lists for $1699.
The 16" MacBook Pro, with 16 GB of RAM, a 512 GB SSD, and a non-binned M1 Pro chip costs $2499. (I think the 16" MBP is a better comparison than the 14" MBP because the screen size is jumping about the same amount as it would between the 24" iMac and the hypothetical 27" iMac Pro.)
Thus, the cost the relevant changed elements—a 3" increase in screen size, an increase in screen quality, and moving to the non-binned M1Pro, costs $1050 ($2499-$1449). The same increase from the 24" iMac would set the cost at $2949 ($1899+$1050). That's why I think the hypothetical 27" iMac Pro will start at around $2999. It could go lower with a binned M1 Pro, but I doubt they'll offer such a config.
Jumping to the M1Max with a 32/1TB config increases the price of the 16" MBP by $1000, so if it does the same for the hypothetical 27" iMac, that config will list for $4K. Since this machine is likely low-volume, I would not be surprised if the price is bumped further to your $4.5K. But it will be in that ballpark. A Mac mini/27" Display combo will likely be in that ballpark as well.
(This is why I decided to buy a 16" MBP rather than wait for the 27" iMac. Again, I'll be happy if I'm wrong.)