I totally get your concerns. They are valid.
However, let's not pretend our bullpen was good last year, just because we ended the season doing well. Our pen was mediocre at best this year, and we are losing Ziegler, Uehara and Tazawa. To me, our pen is more important than out 6th starter. Putting so much trust in Kelly, Barnes, Ross and even to some extent Kimbrel is a big mistake.
Now, maybe we won't look to stay under the luxury limit this year, We may look to reset next year, but assuming we are trying this year, we won't be able to totally repair the pen with just $10M, or we'll have to trade something very good to get a low cost good pen arm.
There are ways to keep Buch and build the pen, but it's going to cost us.