Tatum is probably going to land in the top 25 all time and is a shoe in for top 50 barring serious injury. He will also be a HOFer at this pace. 5 more all-NBA teams, 6-10 more ASGs, another championship and a dark horse MVP possibility are all very reasonable expectations. He's getting into Pippen, Barkley, Rick Barry, Jerry West territory there depending on his longevity.
Brown is a very fringe HOF candidate right now and needs 1-2 more championships, 5+ more ASGs and 3+ all-NBA (or some all-defense) teams to be considered. He'd be in JoJo White or Jamaal Wilkes territory which is enough.
We don't know what we don't know.
Through his age 25 season, Tatum has scored 11852 points, averaged 23.1 ppg, been 1st team All-NBA 3 times (3rd team once), has 67 playoff wins, and won a title. Steph through his age 25 season scored 6814 points, averaged 20.3 ppg, made All-NBA second team once, won 9 playoff games, and had not won a title. I know Brown is older than Tatum and Steph is older than Klay, but is JB that much different than Klay at a similar age (who is listed has having a 70% HOF probability at this point)?
I'm not saying the Celtics are going to turn into the Warriors, nor am I suggesting that Tatum will raise his game and win back-to-back MVPs in his next two seasons like Curry did. But it is possible that the Jays keep improving and the Celtics keep winning. But until we get there and see how things actually play out, we could hypothesize just about anything (good or bad) for this Boston bunch. Tatum is more points away from ranking in the all-time Top 25 in scoring than he has scored so far, so really hard to tell where he will end up.