Surely they could, but at that complexity level, they wouldn't put the necessary amount of effort in it without being payed, and at that point, it's better to hire them.

> but those manuals do not exist - or if they do, they're proprietary.

And there are market-related reasons for that, it's not done because of some arbitrary paranoia. Another important issue is that good documents are hard to write - with regard to driver coding, it's much easier to make a quick call or message the hardware people about some unclear aspect of chip's operation rather than go through the formal process of modifying the official documents. Waiting for external developers to reverse engineer that is slow and leads to serious competitive disadvantages and AMD is an example of it.

The assumption that no good software will be written without pay is outdated as FOSS disproved it many times over.

I think I made it clear that the necessary effort has to measure up to the complexity level, modification volume and time constraints typical for competitive GPU hardware - hasn't happened without pay for any GPU.

That may change if most of the GPU drivers are moved on-chip, which should have happened earlier but there's a lot of politics involved there too, so who knows.