Rational, yes, but rational only in the sense of "the market can stay irrational longer than you can stay liquid".
My experience is many MBS and finance people, during 2003-2008, knew that a ton of debt was garbage, that LTCM's blowup in 1998 showed massive structural weakness that was never repaired, and that a crash was going to come SOMETIME.
But the problem is that the rational decision in such a market is to keep investing and making money on the irrational rise, because no one knows when the crash will come or how bad it will be. It's actually rational to keep buying trash debt when the crash timing is unpredictable.
In the end, the finance people were actually correct to keep buying –– the gov't (that is, me you and every US taxpayer) bailed them out, and most of the traders did come out ahead after a bit of time.