I kind of agree, although I'm not sure it's fair to say that the problem with Android is that you can't easily replace it with another OS. That's not really an
Android problem.
It's incredible what a smartphone can do given its form factor and a lot of that is thanks to their use of SOCs. I have no experience with OS development for SOCs, but I hear it is much more involved because a new version of the OS must be created for each SOC - specialized to work with the device tree supported by that chip. As I understand, Google doesn't do that work. Manufacturers have to fork Android and implement support for their SOCs on their own, then they have to maintain that fork as new Android releases keep coming. It's no surprise then that manufacturers don't want to invest addition support into other operating systems like LineageOS.
There's probably a better way to do things. I'm sure manufacturers could make information more available to OSS communities which would allow them to do the work themselves more quickly and effectively. Like you mentioned, standardization would also go a long way towards making our current smartphone ecosystem more friendly to third-party OSes. But ultimately, none of that is really Android's fault.
Even without Google's vendor deals, I doubt the likes of Samsung, Motorola, or any other major smartphone manufacturer would start supporting LineageOS. It's hard enough to even get Linux suppport from desktop/laptop manufacturers. LineageOS is a really amazing project, but I don't think it's the one paving the way for open source operating systems on smartphones. I think most of that work has to come from the hardware side with projects like the PinePhone.