I think your assumption of all 3 of Brown, Tatum and Smart is way too rich a deal and Ainge won't give up all three.
Either Smart or Brown will be on the team next year if the Celtics trade for Davis.
A point guard not named Rozier(Darren Collison, Patrick Beverly, Ricky Rubio, Jeff Teague, Derrick Rose are all possible),Brown, a healthy Hayward, Davis and Horford can be enough to contend.
I very much hope that Ainge won't trade all three of those guys to get AD.
As for whether the team you mentioned would be enough to contend, I disagree.
I just don't believe a team can be a serious contender for the title with a mediocre starting backcourt.
On top of having an average-at-best lead ballhandler, that team would need either Hayward or Brown to turn into a legitimate second scoring option from the perimeter. The Utah version of Hayward probably counts. Is he going to get back to that level next season?
Horford, Davis and Hayward would be best frontcourt in league right? I think Rubio/Beverly and Brown is not a terrible backcourt.
Kawhi Siakam Ibaka and Gasol will have a field day against that front court. Philly destroy them too
Craziness.
We handled Philly this year with a front court rotation of Horford, Baynes, Morris, Tatum, and Theis, but somehow keeping most of those pieces sans Tatum and maybe Morris and replacing them with a top- 5 player is going to change that?? Get real.
As for Toronto, again we played them even this year but somehow by significantly improving the front court we’ll get worse?? 
100% agree jp. TP4U. Could not have said it better myself(well, maybe without the "Get real"😏).
You guys sound like the Houston fans last year that claimed they were definitely better than the warriors and only the injury to Chris Paul stopped them. I concede that Hayward and Davis could have some success against Toronto and philly in the regular season but not in the playoffs.
You think Hayward/Davis/Horford would struggle in the playoffs vs the Raptors or Philly? Just making sure I’m hearing this clearly.
Hayward has struggled mightily against Kawhi while in Utah. Embiid has destroyed Davis last few times they played and Horford is ageing.yes I think they will struggle in the playoffs against Philly and Toronto.
Ok, so Kawhi locks up Hayward, whose stopping Davis, Brown, And Horford?
Now philly?

? Aged Horford has destroyed Embiid [dang] near every time lol. But for your argument cool, so
Embiid destroys AD, so we switch Horford on Embiid and now Davis is guarding..who? Simmons?... who wins Simmons or AD? Mind you Simmons can’t shoot so AD can sit in the paint and even help double Embiid if needed...
Like you guys don’t think this stuff through.. like I really don’t understand some of the confusion y’all be having.
Hayward/Horford/Davis
As much as you don’t want to admit it for whatever your negative reasons are 3 people who CAN make the “East All-Stars”. Horford old? Well he sure looked great last season.
Hayward hurt? Well he had 1 full offseason injury free so we will see.
Davis?? Idk what negative you could possibly come up with regarding Davis tbh.