In https://letsencrypt.org/2020/11/06/own-two-feet.html, you say:
Can we get another cross-signature? We’ve explored this option and it seems unlikely. ... , the Android update problem doesn’t seem to be going away. If we commit ourselves to supporting old Android versions, we would commit ourselves to seeking cross-signatures from other CAs indefinitely.
But maybe you could consider to get a new (and the last) cross-signature from a root CA that expires far from now, e.g. after 2030, when possibly the market share for Android <7.1 lowers considerably from 33.8% and their traffic gets from 1-5% to some (hopefully) negligible value (the numbers in the previous paragraph come from your article).
So, if any root CA is willing to assume the risk that involves signing a cross certificate for LE, would you be willing to re-explore this option?.