Yes because even if they hit their best case they are both like 5 or 6 years away from even entering their prime and there is no guarantee they will be Celtics or have a team good enough to win a title, whereas if you added Davis to Hayward and Irving, that is a team that is good enough to win a title this year and at any time over the next 5 or 6 seasons.
But in 5 or 6 years J and J will be the stars of the team while hayward and kyrie are hitting the tail end of their primes... that would net us continued success.
Maybe, maybe not. Under your scenario, is Boston going to pay max contracts to Tatum and Brown, when it still has Hayward and Irving making max contracts? I can't see a team surviving with 4 max contracts, so at least one of the older players won't be there (if either is). 6 years from now, Tatum and Brown are both on their second contracts. Who says, they stay for a 3rd or that they are even worth a max extension when their 1st is up (maybe they are later bloomers and thus aren't even on Boston)?
If the premise was next season they will enter their prime as their best case scenario, that is completely different then projecting them to be the best case scenario with the more typical prime ages of 26-32. They are both realistically a long way from their prime. A lot can happen between now and then. And what if their best case scenario is Carmelo Anthony (Tatum) and Paul Pierce (Brown)? Is that type of pairing a championship pairing anyway?
If you have a realistic shot at creating a championship level team for some future unknown, you absolutely have to do it every single time. If it is Brown, Tatum, and salary filler for Davis, you absolutely 100% do that even if you know for a fact Brown and Tatum reach their full potential. It is a no brainer.