I'm fine with keeping him, even being somewhat skeptical of the longevity of RPs. For starters, there's a very good chance that his value will only continue to increase with more exposure, as long as he is consistent. It's possible that he flames out a little bit, but I really doubt it.
Packaging him with Cole is definitely condensing assets, and could backfire significantly. In a situation where we are fully tearing it down, then getting a huge ransom from the Dodgers (like Buehler, Verdugo, ++, maybe including Alvarez) is appealing, but I think you are right overall. I'd also add that there's just too much tied up in this team for 2019/20, and Rivero is still really part of that. The idea of completely blowing it up and moving Marte, Polanco, Taillon, etc etc in addition just doesn't make a lot of sense.
For me, I am dealing Cole if I can get two good prospects who will step in right away. I worry about him going down for TJ more than anything else. Then, they have to figure out the move with Cutch. I don't hate every scenario where he stays, but 5 years is a little bit too aggressive of a commitment, and almost definitely will backfire. If he compromises with a 4 year deal, or a 3 year deal with a big mutual option or something, then I could personally live with it, just because I don't see any way forward for this team that doesn't include really trying to go for it in 2019 and 2020, and I don't know how much trading Cutch vs. taking the chance that he can stay a superstar really does for that situation. Certainly he'd get an ok haul right now, and it would be better than losing him for nothing or the even worse scenario where he is getting paid relatively huge money and sucks again, but there aren't great alternatives really on the table. Polanco and Meadows are question marks. Frazier would make things too crowded, even assuming Meadows is used as some kind of trade chip.
I don't envy the decision and I lean towards just taking the best deal I can get for Cutch this winter, but the focus has to basically be on a kind of combo re-tool/re-build. Everything is compounded by the problem that we will not turn to the FA market to take a chance on the re-tooling part, outside of bargain deals and the like.
In any case, I think the clearest move is to get whatever value we can for Cole. If a deal of 2 of Frazier/Adams/Sheffield/Andujar just isn't in the cards, then I'd be ok with waiting til mid-season, but otherwise I cash out and then re-assess the situation depending on Cutch's market. Any four of those guys would be contributors for 2018-2020 anyways, and ideally whatever would come back for Cutch also is. That's about the best half-plan that I think can be crafted given the options realistically on the table.