I know everyone seems to discount what I've been saying for years regarding top end talent (in large part because it is me saying it), but if you look at every champion since the 70's every single 1, except the 04 Pistons, has either a clear cut top 30 player all time and/or a clear cut top 5 player in the league as their best player (and all but a few that guy is both). EVERY SINGLE ONE (except that 04 Pistons team). So when I say you need a truly special player to win, it is because that is historically what it takes.
Thus far, Tatum has not reached that level and that is why Boston hasn't won a title and likely won't, unless Tatum reaches that level. That is why for years I've advocated making trades that maximize Tatum (or a few years back was trying get a player better than him), which is also why I liked both the major moves this summer ad both KP and Jrue are good compliments next to Tatum.
Also, historically very few teams have had the level of success the Celtics have without a top 5 player (and early on without a top 10 player), so when I say things like Boston has overachieved its talent, that is what I mean. Really the early 00 Pistons and the Reggie Pacers are the only teams that compare historically over a 5+ year period. Obviously thr Pistons broke through fornthe title exception and the Pacers never did.
Tatum hasn't played up to the level he can yet this year and Boston is the best team, so that is a good sign. If Tatum gets on a roll in the playoffs, Boston will have it best shot at a title since 2010 and may even be favored to do so (Denver worries me, but everyone else does not).