I think the better question is can they afford not to extend him? The contracts they've signed have a pretty clear window through 2026, and while they want to have the talent pipeline to not start over afterwards, the roster construction creates urgency to not tie a hand behind your back. Letting Stroman walk(for nothing or very little in trade as you mention) is a big step back for a team that isn't currently at the level they want to be for that window.
Yes, they can use their resources to backfill him in the rotation, but they probably should be looking to do that even if Stroman stays. A Stroman extension is unlikely to be more than a couple million increase in AAV, and they would clearly have the room in the rotation for it given that Wesneski has not seized a role, Hendricks is a FA, and Smyly isn't someone you set and forget for 30 strong starts. You do gain Stroman's salary back if he leaves and that would help, but considering that good, high strikeout SP are desired by 100% of MLB teams every offseason, even if you have the resources to sign/trade for two, it's a difficult thing to pull off in a single offseason. Banking Stroman now makes it easier to get your guy as the hopefully over the top addition.