Blaming original devs for issues caused by a fork can be easily prevented by adding a popup and ignoring anyone who isn't using the official branch. The API works great, I've used it myself to create some things but it doesn't provide the amount of flexibility that source code has. Try taking a look at Minecraft, specifically Spigot which is a similar software with core features as well as plugin support. It's open source and has been forked numerous times for more specific use cases (PaperMC) and has over 500 contributors with performance enhancements and other QOL features that would've otherwise been overlooked. Even though more people use forks of their software than the official branch, there are barely any issues with plugin incompatibility or bug reports going to original developers. The people know that it's the responsibility of the fork to ensure plugin support and fix any issues.
More than anything, I think the largest benefit would come to performance. You seem to underestimate the impact that leaving the code open for a year or so will do. When developers go to study parts of the software for their own reasons, any imperfections in the code will be hoisted. Even having a multi-million dollar software engineering team won't get you the same performance as open-source with a large community (LSPDFR already has, just isn't using it to it's full advantage).
LSPDFR is a massive performance killer, running it solo (no other mods) with very high texture quality will 10/10 times cause texture loss within an hour. It also halves my FPS (from about ~140 to ~80). I'm sure the developers have looked into it extraneously, but so did the 6-figure GTA5 developers with multiplayer load times.