Larry Bird went back and played in college after the Celtics drafted him back in the day. I don't see this as a terrible thing if he wants to go back to college but the Celtics keep his draft rights. They just sign him right after the college season is over.
He is probably going to play most of the season in Portland anyway, with the Celtics paying him, maybe even on a 2-way deal. I can see how it would be better for him take the NIL money and play in College.
I think the two detriments are 1) he's not being developed by the Celtics, who most likely have a specific plan for him that presumably conflicts with St. John's, and 2) there's at least a theoretical chance that he could significantly improve his game and decide to stay in next year's draft, costing us his rights completely.
As an aside, imagine if we'd drafted Bird, he went back to college, and then decided not to sign with the Celts and was redrafted #1 or #2 overall in 1979? Larry would have been a Laker or a Bull, and we would have been screwed.
Compounding matters was that we'd traded the 3rd, 9th and 21st picks in 1979 for Bob McAdoo. We could have come out of the 1978 and 1979 drafts (in which we'd had five #1s combined) with Freeman Williams and a declining McAdoo.
Instead, Bird signed, and we ended up exchanging McAdoo for two future #1s (one of which turned out to be #1 overall) which Red parlayed into McHale and Parish.