I've been flying R models for the last 9 years and had heard that they wanted to replace nearly all of the tails (450+) over an extended 20 year period. As Fatboy alluded to, it is not really the size of the tanker as much as number of booms in the air. We still do sit quite a lot of alert stateside (SAC really isn't dead in spirit
and despite the current love-affair with RPAs, we CANNOT go to war without that many fewer tankers.
The 135 was built for long SAC runways, so anything would be an improvement. I agree it is probably the ideal size as compared to an A330 or B767, but I can't begin to fathom the sanity of trying to rebuild the original. Don't get me wrong, I love the plane, but it is about 50+ years behind in technology, even with the latest B45 to come out, it is nowhere near as efficient ones today.
My 2 cents-
KC
NKAWTG