That's disappointing, but it was also expected. Once I learned that it was a mutual option for next year (i.e., both sides could exercise it or decline it), it was pretty obvious that Bader was going to opt out. He did quite well for the Fightins after they acquired him at the trade deadline. He out performed offensively versus his career averages. So while that made him someone the Fightins would have loved to retain, it also makes him quite a valuable FA. And at the age of 31, this may be his last chance to cash in for a lot of money.
As recently as the 2020 season, he was in the pre-Arbritation MLB status, so he only made $590K. In terms of making big dollars, that didn't happen for him until the 2022 season, when as a FA he signed for 2 years at $10.4M per year. He got $10.5M as a FA in 2024, but "only" got $6.25M as a FA in 2025.