Couldn't it be possible to change the proposal very slightly, allowing only 1 MB for legacy transactions (P2PKH/P2PSH), and to make the 2 MB change exclusively for P2WPKH/P2WPSH transactions? It that was possible and accepted by the miners, the most worrying problem would have been solved.
I think that the idea is exactly the opposite: segwit as a toy for those kids that want to play with it, but "normal bitcoin" needs more room. So essentially, this is, I think, mainly seen as just a 2MB upgrade, with the concession that those who want to play with segwit, get their toy, but that normal old style bitcoin transactions can continue. Once they see that they can get 2 MB, whenever the 2 MB are full, they can do it again to go to 4 MB. In fact, like they used to in the past: augmenting the real block chain room. And yes, if some people want to go to segwit style transactions and addresses, this is admitted that they can do so if they want to.
This is maybe also why the short time frame is not a problem. After all, changing the hard-coded constant to 2 MB is not a lot of work. And whatever happens to the twits that want to use segwit is their affair. If that code doesn't run very well doesn't matter much ; the normal bitcoin part will run OK.