Yeah, possibly. For those with 1.5 and root that are running 2.1 ROMs, they could dumb down the 2.2 (if it truly has some feature beyond the capabilities of the Eris) and get the important stuff out of it.
i think that's what will end up happening. The G1 does not have an official 2.1, but with root, users have access to 2.1 features like Google Maps and all.