It's not just the OS makers. They're also responding to the demand of companies and governments to control their users through them. They will not say "no".
I don't believe that entirely. For example, how much safer is a banking app protected by play protect, running on an OEM ROM with tonnes of OEM/Google/Meta malware, compared to the same running on Graphene, Lineage or Calyx? I think it's the other way around. Google or their associates convince either the banking firms, or more likely the security audit companies that the play protect (safetynet or whichever latest flavor) is an absolute necessity for security on android. In the latter case, those security firms will give the developers a checklist to follow, which will include an item on enabling that API. It's unlikely that so many banks will choose them on their own accord like that, even if a bunch of them insist on Google providing it. I have even seen banks disabling the API in their apps through updates. And they also don't have any problems with their web applications that don't have anything similar to remote attestation. Besides if you look closely, it's in Google's interest, not the bank's interest to enable these APIs. Such apps will only run on the OEM ROMs, making the open source and custom ROMs somewhat untenable.
Its a very slippery slope that is very close to being implemented. In a way, we can hope that the current political climate somehow decimates the American corporations that control the systems, but it looks more like IBM during WW2 supplying counting machines to the Americans and to the Germans and everyone else.
The phone platform is officially lost at this point, there is too much political pressure to control it. We are going to increasingly need to rely on sneaker nets, small mesh networks, and home made "illegal" communication devices. The internet will continue to exist, but it is going to fracture more and more with the political wars that are happening at the moment.
Another approach I wonder about is single task specific hardware, like a GPS unit or media player, what tasks have developed over the past ~18 years within the mobile ecosystem and are mature and not rapidly evolving enough that they can be unbundled to their own devices, and desirable enough to stand alone that there's a market for it.
Or when you do, you can then link it to specific group of people based on the identifiers you received from the attestation.