Busch was graded at 30 for fielding going into this year on the back of playing mostly second base in the minors. The glass half full side of things can point to his improvements and say he's just a drastically better fielder now, the half empty view is that first base is truly that much of a step down, difficulty wise, from a middle infield position, and whatever offensive upgrade you'd get there (depends on the metric: wOBA and wRC look to be substantial step ups, xwOBA not so much) would be given away (and maybe then some) defensively.
I don't know. I feel a lot better about Bellinger than I did a month ago but still prefer all of Happ, PCA, Suzuki, Busch, and Hoerner over him as consistent options. We've tried, and failed, two years in a row to have the offensive plan be 'good all over, great nowhere'....but if Bellinger opts in maybe it's the best route and then you just go sign two of Flaherty/Burnes/Eovaldi and/or go prospects for pitching? Like, yeah, good but not great was the plan going into this year, but that was with us hoping on Morel at third and Madrigal in the opening day starting line up. Whereas Bellinger as your starting right fielder next year probably projects as your 8th best offensive starter.
I'd still rather Bellinger opt out and we get creative on an elite bat. But $60m and a bunch of blocked prospects (behind an offense that's been 5th in fWAR in the second half) gets you at least 2 legitimate starters, some pen stability, and ideally a Carson Kelly. Steele/Shota/Mariners pitcher/Flaherty/Taillon with Assad/Brown/Horton there as support/amped up pen work is significantly improved over where we are now, and there's still plenty of money for the pen and a catcher after that.