It depends on if you think their .500 record last season was an underperformance, overperformance, or an accurate expression of their real talent level.
Given the abundance of injuries, time missed due to Covid, and I think the inarguably bad fit of the roster in general, I think you reasonably have to expect a regression back towards average- average here being the average outcome for a team with a two great wings, one a superstar and the other an all-star, two high-quality bigs with complementary skillsets, an all-NBA defender who's not a complete potato on offense, and a number of roleplayers who may well be able to excel in a limited role or have plenty of room to develop.
A starting lineup of Smart-Brown-Tatum-Horford-Williams actually seems really good to me. It doesn't have any elite playmaking in there, but Tatum's shown real flashes of developing that talent, Smart isn't bad, and Horford and Williams are both rather good at that sort of thing, especially for bigs like they are. If Udoka can instill a good, decentralized and flowing offense in that lineup it could very well compete with the best of them. The problem is that after those five, it wouldn't be outlandish for any one of the bench players, be they Richardson, Dunn, Pritchard, Nesmith, Langford, Parker, or the other Williams, to be worthless potatoes; and if each of them can be, most or all of them could be.
If nothing else, the team quite objectively has two great players and three above starting-caliber guys to go with them, assuming health. Brown is the worst defender in that starting lineup. It's not a bad roster as-is. Timelord would need to be able to shoot threes like a heavily discounted Steph Curry for it to realistically compete with a healthy Brooklyn, but that's fine.
On the topic of Beal as a potential 2020 offseason target, I'm left wondering if it wouldn't be better to go for Julius Randle, or if at least he'd be a great consolation prize. Now, obviously Beal is overall the better player and his relationship with Tatum gives him extra value to the Celtics (given the apparent belief, at least among fans, that in the modern NBA there is a need to treat superstars like some combination of petulant children and arrogant gods among men), but his skillset, being an absolutely elite scorer from the 2-3 position with limited defensive upside, seems to me to be somewhat redundant with, rather than complementary to, Brown's and Tatum's. Whereas Randle, in comparison, still stretches the floor (assuming last season's 41% on 5.5 3PA wasn't a mirage) while bringing a presence on the offensive and defensive interior and on the boards in addition to solid-to-good playmaking, being basically a substantially better version of Horford. Beal, again, is the better player in a vacuum, but is Beal-Brown-Tatum actually better than Brown-Tatum-Randle? I think the answer is at least, "shit, maybe?"
