I'll preface this by saying I like Shota, especially relative to the current sentiment that probably overweights how his 2024 ended(he had a 3.08 ERA into the 2nd week of September!).
That said, the FA market is not particularly kind to 30-something pitchers who sit 90, and they especially won't be kind if they have a QO attached. The only pitcher with an average fastball under 92 who was guaranteed more than 10 million total last year was Trevor Williams, who got 2/14 after putting up a 2.03 ERA/2.79 WHIP/2.0 fWAR in 13 starts.
So from Shota's perspective you have to think of it this way:
* I can guarantee myself 30 million, with an opt out after next year if I end up pitching phenomenally. The Cubs can void that opt out and bring things full circle to getting roughly 3/55 as if this decision was never his.
* If I turn that down, the odds are pretty decent I'm going to get a qualifying offer, at which point I have to choose between less guaranteed money than the option or whatever deal I get with a QO attached, which is very possibly a lower guarantee than my option.
It's not a slam dunk, and athletes like to bet on themselves, but the rational decision is probably to pick up his option.