I think Danny will try very hard to "do something" between now and next off-season, the sooner the better, to add at least one more high-priced, "star" player to his payroll.
The reasoning is he
a) needs to get all of his ducks under wing before Isaiah, Avery & Marcus all hit the end of their contracts in two years. If he already HAS his stars on board, he can use Bird Rights to maintain control over those guys, whether he keeps them or trades them. If he doesn't have another star on board by then, then he has to start worrying about trading their cap space for that of a big star.
b) Horford has a 4 year window during which we want to be contending. We need one more big gun to do so. It would be best to get that big gun now and be able to contend for all 4 years. Next summer would give you 3 years. After that, you've wasted half of Horford's contract.
I'm still inclined to think that Danny is working very hard, right now, on making a deal happen.
Now, it's always possible that no deal for a star happens and we could still get lucky in that someone internally will emerge as a big star any way. Someone like Jaylen Brown could be that rare player the is an impact player from his rookie season on. But that is incredibly, incredibly rare and it just doesn't make sense to put all your eggs in that basket.