GrapheneOS is the only Android OS providing full security patches

Why was it that in the early PC days, IBM was unable to keep a lid on 'IBM compatible', allowing for the PC interoperability explosion, yet today, almost every phone has closed drivers, closed and locked bootloaders, and almost complete corporate control over our devices? Why are there not yet a plethora of phones on the market that allow anyone to install their OS of choice?

You're getting a lot of indirect responses. If you've ever tried to mod your android phone the answer is simple. Its google play services and hardware attestation for things like banking websites.

Its really easy to make a custom rom but hard to do serious "real life" stuff; companies don't want to make it easy. To most regular users, if they cant download apps from the google play store, and they can't use venmo\cashapp, then the OS is dead in the water from day 1

This just shows that the barrier of entry of a new phone OS is more than $0. You can pay app developers to port their apps off of play services, you can pay developers to add support for your attestation keys. Considering how many billions of dollars Android makes for Google, there is a room for a return on investment for an alternate OS to enable investments into a new OS.