I’m not happy about it, but other than adding Lebron or another top 5 player in the league, they cannot compete with Brooklyn for the next year, maybe 2. Ainge needs to move on from Kemba in the off season and add a star that is around the same age as Brown and Tatum. I don’t think they have any other choice but to wait.
Comes down to a question of your priorities.
Are you only bothering to compete in the short term if you can be on the same talent level as the top teams? Or are you trying to maximize the core you have now and let the chips fall where they may?
Are you "title contention or don't bother" or are you "put the best team on the floor that you can given the resources at your disposal"?
Personally I think "title contention or don't bother" is way too narrow of a view. Unless you draft the next LeBron / KD, you're never going to be in a position where you are certain that you're on the same level as the most talented teams in the league. You're certainly not going to be in a position to sign such a player.
And again, if you wait for the way to be clear, you'll be left with just a season or two of Tatum and Brown before it's time to consider the possibility of them leaving. If the Celts are coming up on 1-2 years from Brown hitting free agency, and they still appear to be a clear cut second or third tier playoff team, what reason will they have to think that Brown is going to stick around? What about Tatum?
You could wait for Brooklyn or whoever else to fade and by the time it *maybe* happens, you have to start looking for trade options for your main guys because you can't afford to have them leave for nothing in unrestricted free agency.
The Celts have two in-their-prime All-Star wings locked up for several years. This is the time to try to win. Not later. Forget about what the rest of the league looks like. You can't control that. Once Brooklyn fades, there will be another superteam.