I hope that the Cs are true contenders for the next 8-12 years with all their youth and talent. Ainge has put the Celtics in the best possibly place to do that.
But the contender window always seems smaller than you think. Injuries, cap complications, and improvements on rival teams make it harder than you think to contend.
Just think about the Heat a few years ago. Those guys thought they were gonna win "Not one, not two, not three ..." championships, but more. They won two and they were dangerously close to only winning one.
Just think about the Thunder with Harden, Ibaka, Westbrook, and Durant. They were in the finals one year and it looked like they were becoming the "Big 3s" rival. Then a Durant injury, salary cap complications, and the rise of the Warriors stopped them from getting back to the finals again.
The Spurs were special, but if I remember right, they never won back-to-back championships. Injuries and the development of competing teams always limiting them.
The Mavs might have won another championship if they were able to keep Chandler.
Even the Cs of a decade ago thought they'd make it to more finals series, but injuries and salary cap complications (not keep key role players) held them back.
The Celtics know -- all too well -- that an injury can change a season. They will face difficult salary cap questions each of the next 4-5 off-seasons, and Ainge needs to get every question answered correctly.
On top of that, you have the Sixers who could get James, Leonard, and/or George. The Bucks have an actually NBA coach now and they may try to make a big move. The Wizards are lurking as a possible destination for Demarcus Cousins. The Pacers have a sneaky amount of young players and cap space this off-season that could result in a second (and possibly third) star next to Oladipo. The Heat are a sleeping giant right now until they have the opportunity to make another big move (or two). That's not even talking about the Raptors, or Cavs, or Pistons.
The Celtics would have to go through any of those teams just to get to the finals. Then they have to face the Warriors, or the Rockets (with James next year?), or the Lakers (with a combination of James, Cousins, Leonard, or George?), or the Pelicans, or the Jazz (who are one wing star away from being contenders), or the Wolves (who will be shopping Wiggins for a legit 3rd banana). Even the Nuggets and the Clippers are scary dark horses if they're able to get another piece or two.
I'm obviously not panicking, but I do think we sometimes overestimate the size of the contention window.
That's why this season can't be gravy. It is so hard to get to the finals, and it is even harder to win them. With the Warriors looking human (is Curry healthy?) and the Rockets looking exhausted (is Paul healthy?) and the Cavs on the ropes, and the Cs in an excellent rhythm defensively, hopefully the Cs treat this season like it is their only chance at an NBA finals, because it might be.