What if backward compatibility had remained the standard on post-Crash consoles?

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In regards to your main question the N64 and Saturn probably would have sold better if they had backwards compatibility assuming the console costs would remain the same, though the N64 didn't exactly sell poorly overall. The Saturn would have sold better if it was fully backwards compatible with at least the Sega CD, though much better if it was fully backwards compatible with the Genesis and 32X. At least with the Sega CD weren't those games on clearance when the Saturn was out? If so those would have been an easy cheap library for people who never spent the money on Sega CD hardware previously, personally I'd find that enticing while waiting for the latest and greatest new Saturn games to come out, all this before people were burnt out on trusting Sega hardware. The biggest reason that affected the sales were the competition, if the competition wasn't so good then the Saturn would have done better even without backwards compatibility. Not sure about the Dreamcast though, people were burnt by the Saturn so few people trusted Sega by that point, plus the competition with the PS2 was just too good, that and Dreamcast games and controllers mostly sucked in general. I'm sure if the Dreamcast had much better games then it would have sold better too before the PS2 launched.

Agree 100% (except the Dreamcast did have good games), backwards compatibility would have been a huge help for the Saturn. I'd argue it's the system that lost the most from not having it. For one, the Saturn launched the year before the N64, and backwards compatibility helps consoles the most early in their lives, before the new console's game library builds up and the old generation gets too outdated. Certainly with the 16-bit generation still being very current in 1995 and the Saturn's early lineup lacking, these older games could have been an important bulwark for the system until better games came along after programmers started to figure out the system from 1996 on.
Genesis compatibility would have helped immensely, but even just Sega CD compatibility would have been a lot of benefit.