Well, that's precisely the thing. Within those three years, Tatum could absolutely be a MVP type player, and he's already made it past the Bucks as a part of another team. Durant and Harden are both on the wrong side of 30,and Durant is coming off one of the most debilitating injuries a player can suffer.
If Tatum makes another leap -- and I think he will, and if Jaylen makes another leap as well -- to not have strengthened the roster precisely at its positions of weakness would be a wasted opportunity. Two of the three Celtics starters -- Kemba and Robert Williams -- are prone to injuries that could take them out of substantial chunks of the season and the playoffs. Replacing Williams with TT is not the end of the world, but it does force the Celtics to play Grant Williams and Kornet in backup minutes, and that is far from ideal. Watching Kemba fall off a cliff, however, would put this team in a huge hole. The team needs a playmaker to accompany the Jays very badly, and unfortunately they may have to ship Kemba out to be able to afford it. The Horford-Kemba trade remains one of the best options out there, I think, and one of the few that makes sense.
Regardless, to not try to upgrade at one of those positions, given that Kemba is a ticking time bomb and Robert Williams is fragile as hell, would be to potentially waste 2 or 3 years of potential greatness.