I don't think there is anything wrong with expanding - just as with the football playoff when it was 4 teams, we see teams on the wrong side of the bubble that were certainly just as good and deserving as the last few teams selected.
That logic leads to expanding it to every single team.
You mentioned FBS so, let's compare. There are ~134 teams where it used to be 4 and it went to 12. That is still less than 10% of teams make it.
There are ~350 D1 basketball teams and 68 already make it. That is right at about 20% of teams making it.
I am not (necessarily) arguing against expansion of the basketball tournament, but I don't think the FBS situation is a good argument for it. If anything, basketball is an argument for expanding FBS (ignoring the typical difference in rest times between games in the two sports).
The point of my earlier post was that it shouldn't be the coaches that determine if it gets expanded or not. I think it should, ultimately, be the fans via how much revenue they generate for the tournament. If it get's too big, I am assuming it will just water it down and kill interest in the whole thing, or at least the earlier rounds.