Neither pitcher was on my radar for 5 years, that's how long they went between effectiveness. But Boyd has never reached the ceiling that Flaherty has and Flaherty actully showed that he can get back to that ceiling across a meaningful sample of innings, whereas Boyd, for all intents and purposes, pitched the best baseball of his career (largely because of a career-low HR rate) across 50 (with postseason) IP. Boyd has a 4.6 career FIP and hasn't cracked 80 IP in 5 years. That's actually an incredibly risky gamble to give him a rotation spot and 15M, even considering Flaherty's injury history.
It does appear that he is waiting out the Bregman market though and I'm a big Bregman fan so it's whatever.