I get the impression the Geneve also suffered another issue the C-128 had: It was more like two computers in one box than it was an upgraded version of the older computer. Quite literally in the 128's case.
You can't easily make software that runs nicely on the newer system and gracefully downgrades itself on the older one, in the way you can on an IBM-compatible or a GameBoy Color. You can't choose to make Geneve or 128 software that degrades gracefully on a 4a or a C64.
The 99/8 was (more or less) just a faster 4a with more RAM, so there was a lot of potential for software sharing. Just as a lazy example, a TI-Writer II with larger(or even multi-)file support on a 99/8 that ALSO ran on a 99/4a by dialing back to TI-Writer I file behaviors, while still benefiting from any other changes.
You can't really do that with a C128 or a Geneve, because the new computer behaves so differently outside of compatibility mode. .
ON THE OTHER HAND, the 99/8's graphics capabilities would have been a serious sore spot. The 9918a was looking pretty dated, and my impression is that the 9118 didn't actually bring anything new to the table.
I started this thread with a comparison to the C65 and the project to basically re-create and re-invent that machine. Commodore's last 8 bit got nearly to production, but not quite, and came very late in the game - 1991. It was a wholly different concept from the earlier 128, although it was to have, like the 128, a 64 mode. But whereas, as you say, the 128's 64 "mode" was in fact just a second 64 computer inside the 128 box, with a dedicated processor and everything, producing 99.9% compatibility or something like it, the "Commodore 65" would only have emulated the 64 in software, resulting in a compatibility of 75% or so (which I have never really understood what that means - it seems like it could mean that like all of the software would be 75% compatible, or, to put that another way, not compatible at all!).
The issue about compatible software will exist for this new machine just like what you are talking about with the TI-99/8; since the REAL C65 was never released, there is NO software for its native mode. Yet these folks at mega65 are doing it anyway. I must say the Commodore 64 software/hardware scene is rather robust for a machine dead 25 years now, and I guess that is not maybe the case with the TI-99 scene, but it seemed like it was from a (mostly) outsider's perspective.
The 65 was to be, in other words, quite a different machine, and of course it has long since reached mythical status amongst Commodore fans. I was so impressed that there were actually people developing a latter-day 65 - the time involved seems just enormous - but it made me wonder if any other retro communities had similar project developing - or if other retro communities would even be aware of just how far these Commodore guys seem to think they can take it.