Things you do not know - the performance of the 2019 bullpen.
Tyler Thornburg could very easily outperform Ottavino next season and it wouldn't be shocking whatsoever.
Yeah, every year there are tons of pitchers who come out of nowhere and have very good to elite seasons in the bullpen. The Sox even traded for 2 of them in Carson Smith and Tyler Thornsburg. That didn't work out so well. There is just so much variance in relief pitching I don't think it is worth paying money for. They need to do a better job developing bullpen arms... or really just pitchers all together.
Next year, the board will want to sign a reliever or two from a group of 5-6 pitchers that none of us want right now. This year, we look forward to Lakins, Feltman and Darwinzon to possibly contribute and it will end up being a lesser known like Jake Cosart who has a big time arm but 0 control. He made some serious progress this year that is masked by a bad start, bad luck, and last year's good luck.
Cosart 2018
First 13 appearances: 8.49 era, 23.1 ip, 11bb/27k, 6 HRA. .297/.366/.594, .353 BAbip. 24.1% K rate/9.8% BB rate
Last 24 appearances: 2.87 era, 37.2 ip, 13bb/50k, 0 HRA. .247/.311/.317,
.365 BAbip. 30.9% K rate/8.0% BB rate
and only AA
First 11 appearances: 8.41 era, 20.1 ip, 9bb/24k, 6 HRA. .279/.347/.570, .321 BAbip. 25.3% K rate/9.5% BB rate
Demoted to A. Called back up 2 months later
Last 8 appearances: 2.81 era, 16.0 ip, 6bb/21k, 0 HRA. .259/.324/.293, .375 BAbip, 30.1% K rate/8.8% BB rate
2017: 3.10 era, 38 g, 49.1 ip, 41bb/52k, 5 HRA. .163/.335/.291, .195 BAbip. 23.4% K rate/18.5% BB rate
2016: 1.78 era, 37 g, 70.2 ip, 36bb/104k, 2 HRA. .172/.280/.260, .283 BAbip. 35.9% K rate/12.4% BB rate
I'm not saying it will be Cosart specifically, just that there are a lot of pitchers on the farm with potential to be good or even great bullpen arms.