My argument is that Bellinger's actual tools (physical athletic abilities) aren't any worse even if some of the statistical results have been worse. Launch angle is not something largely determined by his raw physical tools, it's about mechanics, timing, intent etc. This can change year to year and the stats can go up and down while his physical tools remain the same, which suggests he can possibly improve on this next year, and probably overperformed last year compared to what we should expect in an average year from him.
Similarly, baserunning value can go up and and down year to year for various reasons, but his sprint speed is the same as last year (actually ranks better this year) so is there any physical reason for this decline, or it is just some random variation, or changes in another variable outside his control like base-coaching (more or less aggressive sends etc)?
I don't see how his defensive abilities have necessarily changed either even if the stats aren't as good as last year. He still has the same sprint speed and the arm strength is similar. Defensive metric outcomes will vary year to year based on numerous factors including natural variation (luck) due to limits in sample sizes so a decline this year isn't necessarily an accurate reflection of a player's abilities declining, especially considering Bellinger only played 80 games in the OF last year and at different positions and will likely have about the same # of starts in the OF this season too.
I'm a big believer that teams should largely pay for tools (and skills that take many years to teach) and focus on evaluating those properly rather than getting overly reliant on the results on the field in a single season since those are far more variable. I think the best front offices are able to best separate those and evaluate players most accurately.