Not sure I'd make the decision to go for it in their position either, but do they really have much choice?
No one will take Cano, Seager, or King Felix (and give you much without making you eat all their money), and you're going to have to get the fans to swallow awfully hard if you trade away the controllable talents like Paxton, Haniger, Segura, and Diaz you have to move to get lots of high-end talent back.
Not saying they won't do it, but will Seattle fans put up with a deep rebuild, AND wouldn't you think they'd have already replaced DiPoto if they were going to try it? He's not exactly a rebuild-type GM.
Kinda hard to say they couldn't spend a chunk of money this winter and not at least be in the next group that's hanging around in the event any of the expected contenders have injuries (or fall on their faces like Washington did this year). I loved the run that Oakland made as much as anyone did, but how many people honestly saw that coming given their lack of starting pitching? I can't recall the first person including them in the postseason conversation until they were well into their hot streak.
I don't disagree that they aren't likely going to pose a threat to the big boys in 2019, but if they splurged a little on one SP (Span's $9 million can be jettisoned, Juan Nicasio's $9 million comes off the books, letting Zach Duke and Adam Warren walk saves another $5-$6 million), they've got a couple pieces in Julio Rodriguez and Braden Bishop that they could probably use to go get another back-end SP. Re-sign Maybin as Span's replacement, and hope Lewis shows some improvement and is healthy enough to replace him sometime after the break.
That ought to be enough to keep them around the same level they were this year (if not make them a bit better).