There's also the fact that if a certain deck type becomes too popular, everyone will start using a deck which specifically counters that type and it will become less effective, thus there will be more of a reward anyway for coming up with strong but original decks.
Except that nobody really figured out a true counter to buffed-up Rainbow, Hermes AND Scorpio...
in theory, a good DBH can beat anything except immodecks. A good DBH with AMs can even beat immodecks, but I don't (yet) know how to squeeze those in...
Nope. First of all, Sanctuary is going to be common in Rainbows, because it answers well the most direct counter - Black Hole. Why skipping what would be good protection for nothing?
Second, upped environment has definitely more answers for DBH. Upped quanta producers and low cost PC is enough to stop DBH, as well as most shields, and with a 3x Mark you're going to consistently see those.
Third, DBH severely lacks damage. It always did, even with upped Amethysts. Having a 200 hp opponent means you're very much likely to not have enough BHs to stop them from starting, and DBH is all about denial, can't really put up a fight without that.