There's no magical time at which Bitcoin suddenly becomes "mainstream" and then people have to work with what's in front of them.

The reality is that BTC adoption will be a slow process which will happen based on both the security and the convenience on both chains.  It's most likely that there will be several different solutions implemented in an attempt to increase capacity without only increasing the block size. 

In the future when the blocks are full again, I don't know what will happen, but that's because the solutions are already going to be worked on.  If it desperately requires a block size increase, that'll probably happen.