The problem with promoting an OXP station (or dockable) to main-station status is that they are not guaranteed to be there after a save, and main stations are the only place you can save. It's a bit of a conundrum!
Ah, yes. But that would propably be possible with extra scripting, though? (Oh, and with a warning not to remove station oxps before there is a new station...)
What would be nice to have, but it's way above my abilities, to script a general desaster oxp, where the main station is built up again over time... first a smallish not much more than a dock thing replaces the main station (and still beeing there if another OXP calls for it) and then it begins to grow, with maybe working drones scuttling about (and essential doing nothing as long as the player is there, exactly like normal construction workers...
That way it would be possible to destroy main stations because of terrorist acts, technical failure, asteroids, solar storms, war atrocities... and after a while everything would be back to status quo. Would even allow players to blow up stations without remorse...