GrapheneOS supports attestation too, so even if they succeed it will likely just turn into a gift to Google, Apple and GrapheneOS. It's hardware attestation that needs to be opposed as it's inherently user hostile, allowing a single popular Android distro doesn't do much in the grand scheme of things.
The more the better - being forced to maintain an up-to-date list of Google competitors (including some that don't keep attestation keys secure, so the bad guys will pretend to be those and you'll be forced to allow it anyway) may make some reconsider whether the feature actually brings any value.
Every Android system support remote attestation. It's part of AOSP. Google just decided not to use it, because Play Integrity allows them to lock in phone manufacturers and force them (per leaked agreements) to preinstall a bunch of Google apps and require to run Play Services and some other components privileged on the system.