First of all because of assertion that each SH-2 must have own 1MB of RAM pool as if it can not be shared.
If there were two 1MB 32bit SDRAM chips as Work RAM then 16bits of each per SH-2 processor.
Even if not shared, there is two-way connection between those two SH-2 processors.
I was suggesting an alternative to the master / slave arrangement that might have worked, though with the downside of added complexity.
You can't just arbitrarily assign lanes from a memory chip to a processor and say job done. It's not that simple,
Asserting that boat has sailed away when Sega choose SH-2 for their CPU is outright false and utter lie.
Not only that it ignores history as Sega Genesis had Z80 processor that was used for audio.
Thus Mega Drive was backward compatible with games made for Master System.
The Megadrive was a different system, and the Z80 had the 8KB of audio ram to use as Master System work ram, with some way of correctly mapping memory for MS mode. It could act as CPU while playing Master System games - there was a specific Master System mode that the Megadrive switched into, effectively disabling the Megadrive only elements.
The Saturn sound CPU appears to see a different memory map to the CPU in the MD, and the Saturn sound CPU seems to be very low priority in terms of sound ram access. Even if Saturn didn't have an incompatible chip, I don't think you could do what the MD did for MS backwards compatibility.
It's not to say that would be the only problem with BC of course. Lacking everything else in the MD was pretty big too, but falling at the CPU seems like a good place to start.
SH-2 apparently could not handle audio chip while SH-1 can process Full Motion Video's on its own.
With or without MPEG decompressor that adds 512KB of FPM DRAM and 512KB program ROM.
Developers already did when SH-1 placed soon needed game data to CD-ROM DRAM cache.
It's not about the performance. Plus where have you seen that developers can run their own code on the CD-ROM subsystem's SH1?
Adding one more MB of FPM DRAM would be 10USD, replacing SH-1 and 68K with SH-2 would reduce costs.
Do note that Saturn did not have second SH-2 nor VDP at beginning of 1994, only by mid to end of 1994.
Adding one more SH-2, VDP and DRAM is very large change to architecture and design of Sega Saturn.
How do you propose replacing these two very different processors, in very different blocks within the system, with different interfaces, with a single SH2?
Sega Saturn would likely, almost assuredly sold at same price it launched even if it had my alterations to hardware.
Ignoring for a moment that some of your alterations seem to be unworkable, yeah they could have launched at the same price but that's because sale price and BOM are rarely the same thing.
I don't think these changes would have made any real impact on sales, but they would have hurt even more as Sega tried to compete on price with Sony.