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 extensi

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.