That was part of it, but there was also a desire to be very sure we had enough address space to avoid needing to go through this again. SEND and privacy extensions also make use of the 128-bit address to add some security.

It's already possible to connect from v6 to v4, and using 48-bit addresses wouldn't make doing so any easier. 48 bits would also be way too small; there'd be no point in going to this amount of effort to update IP only to then have to do it a second time straight afterwards.