My take from reading various articles and interviews is that
a) the choice of all hardware used in the Saturn,
b) the early release of the Saturn in the US, and
c) the conception of the 32X as well as the insistence on it using dual SH-2s
were all things that came from SoJ and were forced on SoA.
Our accounts all come from SoA, so it's entirely possible that we're getting filtered facts. However, if the above is true, then I think it's fair to say that the blame for Sega's failure rests mostly with SoJ. Whatever other mistakes there were at whatever other times, I think it's primarily those three things which prevented Sega from getting the foothold they needed in the 32-bit generation, and that's what led to their bankruptcy.
Furthermore, it seems to me that the only real mistake of its own that SoA made when it was under Tom Kalinske was believing that FMV games would be the future, and investing likewise during Sega's most successful years. The money lost on other fruitless R&D and things like high executive salaries was perhaps unavoidable and probably less consequential.
And after Kalinske? Well, Bernie Stolar was a moron, but by the time he came in, the damage had been done. I mean, having Working Design's translation of Saturn Lunar 1 would have been nice, but it wasn't going to save the system. As far as westerners were concerned, the Saturn had no meaningful advantage over the Playstation in hardware or software, price or potential. The end.