We periodically evaluate the use of the mainline kernel in CoreELEC. It is however not feature complete enough on all supported devices to make a switch to it. While for some use cases it might work ok, that would not be the case for everyone, and we don’t want to be in a position of having to say sorry we are now taking away a pile of features that currently work, because we wanted to make the switch before it was ready to do everything all our users expect.
For general non CoreELEC use of mainline kernel, while not really on topic on this forum, I have both the Le Potato and La Frite boards, and it works rather well on both for general computing non media use.