alt Hacker NewsTI with the TMS9900 was in a similar position as IBM with the 386: both threatened their micro/minicomputer markets. This is why TI didn't release other TI-99-like machines. TI also expressed disdain towards hobbyists.
Edit: It might be cool to make a higher-spec TMS9900 home brew system.
The TI-99/8 looks like it would have been awesome. It solved the issues of the 99/4a, had improved basic and pascal built in, all while being backward compatible.