I just got a 15yr fixed rate at 2% (!) which made me think a lot about what's behind your comment. In particular, what will happen once rates go back up:

1) Right now we are at zero short term rates, and moreover mortgage rates are propped due to Fed purchases of Agency MBS

2) Say rates go up 2% (not crazy) in parallel. So now your 3.5% becomes 5.5% which is still historically moderate. However, the 5441 required monthly income from your formula is now 6877! 26% increase.

3) What then? Prices go down?