> There were Apple II clones with varyingly clever schemes around copying the original ROMs, and it provided slots so many manufacturers could provide hardware for the machines.

It looks like the vast majority of clones used a (near bit-by-bit) copy of the original ROMs. I've only heard of one company using licensed ROMs and one company using reverse engineered ROMs in the American market.

> And let's not forget the ubiquitous S-100 bus, with hundreds of different companies making hardware for. In 1983 you could assemble a computer around your choice of processor, graphics (or not), IO, mass-storage, and so on.

These machines, in conjunction with CP/M, were the closest thing to what the IBM compatible would become, in terms of it being an open platform. Unfortunately, they were also notorious for being incompatible with each other. As you noted, they didn't have to use even remotely similar processors. Heck, you couldn't even exchange disks between many variants (the format was incompatible).

> Entirely accidental. What really started this was the reverse engineering of the BIOS.

Not really. IBM didn't intend the clones, but the effort to create clones was intentional. Not only did you have the outright rip-offs and famously cleanroom reverse engineered ROMs, but you had many companies making machines with various levels of compatibility while trying be legal. Once the PS/2 arrived, IBM's attempt to regain control of the platform, there was a concerted effort by their competitors to ensure they (rather than IBM) defined the standard.