There are no bottle necks 128MB. DASH tested 400MB blocks without a problem. Plus miners control how large blocks are. This is just an upper limit.
Do you think miners will risk getting orphaned with such big blocks? Block propagation could be improved a lot. Also I don't know if mempool acceptance is just an ABC issue...
Miners can increase block size at a rate they are comfortable with. The first version of bitcoin did not have a blocksize limit.
