Did Boston magically grow a top 5 player or even a top 10 one?
Tatum is a top 10 player, Brown is arguably top 15.
I only have LeBron, Doncic, Curry, Jokic, Embiid, Lillard and Harden ahead of Tatum. AD is probably there too if he's healthy.
Kawhi. Paul George. Jimmy Butler.
First is injury-prone / rests a lot, and his defence has fallen off to a similar level as Tatum's. They're probably on the same level, same with Durant (who plays prettttty bad defence). The others, simply no and no.
Durant and Leonard are absolutely better than Tatum. They do miss games, but they won't in the post season. The 2 time defending MVP (who is also the defending DPOY) is also clearly better.. Davis is better as well. That is 11 right there without arguing about Butler, George, Beal, Zion, or anyone else.
The larger point is you really do need a top 5 player to truly contend, unless of course you are one of the 3 champions in the league's entire history that doesn't have one. And Tatum is no where near a top 5 player.
My tiers are something like this, probably forgetting some:
1) James, Jokic, Curry, Leonard, Durant, Antetokounmpo, Harden
2) Embiid, Doncic, Lillard, Davis
3) George, Tatum, Butler, Towns, Beal, Gobert, Irving
4) Adebayo, Booker, Brown, Vucevic, Middleton, Lowry, Sabonis
5) Ingram, Young, Mitchell, Simmons, McCollum, Williamson, Randle, VanVleet, Holiday, LaVine, Siakam, Wood, Fox, Hayward, Grant