I think that for the next couple of seasons, their best shot at winning a championship is by adding Davis, re-signing Kyrie, and rolling with a Kyrie/Smart/Hayward/Davis/whomever lineup. I'm assuming Al would need to be traded for salary purposes in the AD deal. But I don't really know. Either way, I think pairing Davis with Kyrie is the best idea for the short term. And honestly, both are young enough so that they might be the most viable option for the next 7-8 years as well.
But...
I don't know what the future holds for those two. I don't know that you can make a deal guaranteeing they both stick around that long. And it's still a little hard to know how Kyrie would respond to being the 1A guy again, like he was with LeBron. He didn't like that last time, but AD is definitely the better player, even if Kyrie might not see it that way. But regardless, these two represent the most elite talent the Celtics would have had in quite a while.
That said...
I think I could be sold on a future without either of them. Kyrie walks. AD doesn't get dealt. I think that Brown and Tatum are given the room to grow into big time stars, which I think they're both capable of being. Not elite, all-NBA superstars, but 20-24 ppg kind of stars. There's still lots of time to go before either of them hits their primes. Add in Time Lord and some of these new draft picks (hopefully!) and that's a pretty good core.
My thinking is this: Every team wants to acquire talent and make a run at a championship. The vast majority of teams try this and fail. Here's the franchises that have won NBA titles in the past 10 years:
Golden State (3)
Miami (2)
LA Lakers (2)
San Antonio (1)
Cleveland (1)
Dallas (1)
Six teams. There are a ton of teams with elite NBA talent that have never won a championship. For example:
Boston (Kyrie/Hayward)
OKC (Westbrook/George)
Houston (Harden/Paul)
Milwaukee (Giannis/Middleton) - we shall see about this year
Toronto (DeRozan/Lowry or Kawhi/Lowry) - again, we shall see about this year
Portland (Lillard/McCollum) - maybe this year
New Orleans (Davis)
LA Clippers (Paul/Griffin/Jordan)
I mean, teams try and try and most fail to achieve the ultimate goal. And when they fail, it's usually back to the drawing board for a messy rebuild.
The Celtics gave it a two-year run with Kyrie and Hayward. Unfortunately, year 1 they had a great year but injuries took those two stars out of the mix. This year, they were worse even though they were healthy. If they said, "Well, we tried" and let Kyrie walk, they'd be in rebuild mode. But their rebuild mode would still feature a bunch of very very good NBA talent, some of which might be on the cusp of stardom themselves. So if this is a 2-3 year rebuild, this young core is as good a way to do the rebuild as I can imagine.