ZiPS has Bellinger's projection at .267/.327/.441 and 2.7 fWAR. If he does that, he's worth something like $20-25M per season, depending on the $/WAR figure you use.
So this $80M guarantee is probably a bit high. It would, of course, be worthwhile if he's showing the skills to outperform his projections. But if he's doing that, he's also opting out. So in the event we actually pay out the $80M, it's quite possible we are "wasting" like half of that money due to injuries or underperformance.
But I have always maintained that it is incredibly hard to have an albatross of a short-term contract. And Bellinger has the physical tools and upside (and health) that might enable him to vastly exceed the projections. The risk/reward ratio here feels about right, and since I don't have to worry about this thing for the next 6-8 years, I'm reasonably happy with the signing.