I think it is natural that fans find fault with whatever is extant, and want to propose improvements. I'm probably the rare fan who thinks we're at optimum design right now, though I could see a 6 team playoff personally as being OK also.
I personally am against 8, but whatever, my preference is not really relevant. And I am pretty sure we stay at four for years while fans design ever more intricate alternatives and argue about them, while we stay put.
I think everyone would agree that an 8 team playoff would generate more MONEY, which usually drives decisions, and in this case it does not apparently. So, we can infer there are significant negatives perceived by the PTBs about going to 8 to offset the positive of more money.