there are already DX->GL compatibility layers, although most of them are in house, by companies that produce ports (ala Feral Games).
yes, i entirely agree with you.. almost. ideally, i'd like DirectX to.. go away, forever. proprietary APIs that result in lock in to a software or hardware platform are seriously holding back computing in general. and i'd never recommend anyone use AMD's Mantle for the same reason (which is a massive waste of everyone's time)