FWIW, an alternative outcome to lowest-common-denominator is what's happened with, for example, alternative Python implementations: there's still a main blessed

FWIW, an alternative outcome to lowest-common-denominator is what's happened with, for example, alternative Python implementations: there's still a main blessed one, most library authors don't care about whether or not anything works on any of the others, and users who want to avail themselves of the benefits of one of the alternatives (e.g., better runtime performance on PyPy) probably just can't, if the libraries they need aren't supported, unless they go to the effort of adding support themselves. (This outcome is probably also bad, to be clear, but maybe of less practical consequence to people who continue to use mainline rustc.)