I think some folks are too concerned about trading the next Sandberg or Jenkins. While the Phillies shouldn't trade much for a rental like Bellinger, I would trade a midlevel prospect like De La Cruz, perhaps adding in a lesser prospect (Hans Crouse?). If that doesn't get it done, so be it, but it's certainly worth trying, even under the assumption he is gone after the season.
Bellinger has a greater oWAR (2.8) this season than anyone on the Phillies, despite playing only one more game than Bryce Harper (who is at 1.2 oWAR). And he's way better defensively than all of the Phillies' corner outfielders/first basemen (other than when a centerfielder is playing at a corner). That's not a very high bar, but it means a lot of improvement. He immediately improves the team by replacing Cave in the lineup. And he would be excellent insurance if there's an injury to a starter other than Stott, Turner, or JT. He could easily make a two game difference in the standings, which could be the difference in making the playoffs (it would have last year) or help in the seeding.
How likely is it that Bellinger turns into his 2021 self (instead of his other six seasons)?