It's not guaranteed to work out, sure, but is there or has there been anything that can realistically be done this offseason that could be expected to make the current roster into a contender? The most I can think of is try to pull off some Hail Mary by packaging Brown, Williams, and whoever else for Towns or some other top-10, top-15 player, but I'm not sure any such trade that would (assuming it even could) actually be accepted by the other team would be worth it for the Celtics.
What is the alternative? Any strategy could fail, resulting in a wasted Tatum. The reality is that short of the young players currently on the roster, Tatum and Brown included, making substantial leaps (be it Tatum getting 10 FTs per game and becoming a really good facilitator, Brown finally becoming the elite defender we once thought he was, Williams gaining the ability to stay healthy, Pritchard growing three or four inches, etc), there's not really anything that could be done to make this year's roster into something on par with the best teams. So what's there to do other than try to make it so that you're not in the exact same spot next offseason?