I agree – more or less – with the first two of those premises. I would quibble a bit, because how quickly the mutation spreads will depend on how beneficial it is. And the environment could change again so that it is no longer beneficial – before the mutated gene is fixed.
But, okay, that’s a quibble. The general picture seems about right.
However, I disagree with premise 3. Note that I am not a biologist, so I could be wrong about that. But it seems to me that unmutated gene could finish up in the junk DNA, where it is then protected from selection. And, at some time later, it could re-emerge after being transposed to a coding region of the DNA. That is to say, I see junk DNA as potentially being a kind of long term memory that can counter what premise 3 asserts.
On your general point – yes, I agree. If it turns out that we are all devolved (and degraded) bacteria, then we should be celebrating devolution and degradation. Or, to say it differently, Behe’s argument seems absurd.