So in essence everyone is depending on Steemit Inc's large share of stake as a sort of proof that it and its process and roadmap are in the interest of shareholders in general, right?

I can't speak for everyone. People likely have different views.

And at least at this point, things like golos aside, there isn't a large enough reason to fork and do things independent of them and compete against the main chain ...

There have actually been several Steem forks, not just Golos. There are probably a number of reasons why none of them have really gained traction but I'd guess that the lack of dramatic success by Steem itself (at least in the terms that the cryptocurrency market cares about, which is mostly market cap) is a big one. If you fork a #30 coin, how many people will actually care? Yes there are issues with liquidity and such, but a lot of those are secondary to the primary issue of people caring enough to pay attention to a fork.

if, say, steem were totally a near-dead coin and got delisted from upbit or binance etc. and there was more upside potential in starting something fresh.

Hard to say. If you fork a #300 coin, do people care about that more or less than forking a #30 coin? And there have been and continue to be numerous "social-crypto" projects being launched. Again, I would say until someone shows at least one of them being a clear and dramatic success, interest overall will be muted.

submitting game-changing changes to steemit would probably rather take their ideas and skills to another upstart where they have a shot at getting a much larger share of stake in the project, considering that is how steemit itself was kind of arranged at its inception

Probably so. However, even modest improvements or incremental changes don't go anywhere. Steem/it does have a community and some in that community (including witnesses, but not only witnesses) are interested in contributing to the code. There just isn't a mechanism for it.

I don't think we'd get many open-source enthusiasts who code for ideals over money if this is how code changes are handled here with the past processes.

You are correct. The process is hostile to open source involvement.