Years being the top player on the team, by WS:
Paul Pierce: 11 (first one in his first season, age 21)
Jayson Tatum: 0 (he's well behind Walker this year, too)
Pierce was the top player on the team (by WS) each of his first 8 seasons, had a hiatus in the glory years of 07-10, and then another 3-year run from 10-11 to '13, to land him at 138 WS, right in between Havlicek and Bird. Pierce was incredibly durable, missing significant time only once in his first 18 seasons, and was getting starter minutes up through age 37.
Tatum would need to be a perennial MVP candidate for a decade before we'd really be discussing a top-5 rank in the franchise. And quite frankly, he need not get anywhere near that to be in the "jersey retirement" zone, so long as teams he leads win at least one title. And in fairness to Tatum, it wasn't until Pierce's 4th season, his age 24 season, that he became an all-star (but then he was an all-star 10 out of 11 years, leaving aside his half-injured 06-07 campaign). But still, that sort of sustained excellence depends on so many factors. Maybe I can see a window where that's Tatum's "Ceiling", but if ceiling means 95th percentile outcome given data through today, rather than (say) 80th percentile.