This would be rather unfortunate as that would also leave out all 32-bit only AMD processors (Athlon XP & co) as well according to Wikipedia where it's said Athlon 64 was the first one to add SSE2 and it can already run the 64-bit CentOS anyway.
I'm hoping there is more people that could +1 having support for pre-SSE2 CPUs so it would be seriously considered even though it might need massive rebuild of the multilib packages. EPEL doesn't have multilib yet (right?) so they can still adapt to whatever is going to be done. The packages would run on upstream as well anyway.
Taking into account the actual computing power of CPUs, I don't think it's unreasonable to run CentOS 7 on Pentium III or Athlon XP.
Thanks for considering.