While I agree with the timeline, your post isn't complete without anything addressing our financial context; how the team is shaped not just on the field but against the luxury tax.
That's actually the point, IMO.
Cards on the table: I think this roster is probably going to need a pretty dramatic rebuild around 2020, whether we sign another bat or no. The Price opt-out makes all of this something of a garden of forking paths, but looking at the spreadsheet over at Cot's Contracts, here's how the big picture shapes up.
Over the 2018-2019 offseasons, we will probably need to replace 500-700 IP of starting pitching at the same time as Betts, Bogaerts, Bradley, and Rodriguez are getting more expensive. In the 18-19 offseason Pomeranz and (perhaps) Price are FA. After 2019, Sale and Porcello are FA. In 2019, Bogaerts and Bradley are Arb3 — although Bradley has another year), and Betts and Rodriguez are Arb2.
We'll have help because of expiring contracts, but less than you might think. This could be mitigated if a few of these low-minors pitchers (Groome, Houck, Mata, Shawaryn, Scherff, Darwinzon Hernandez) pan out in a hurry, or Price a) stays and b) ages well, or Rodriguez becomes the 1-2 starter that he seems like he could be with a bit more consistency. Maybe one of our position player core gets traded for good value. But I don't really see where all that pitching is coming from. Presumably some of our interesting low-minors players will have blossomed into the sort of prospects that could be useful in trade, but certainly not enough to replace most of a starting rotation, and we won't be able to pay retail to acquire good FA SP if we're extending the kids.
The big moves made since Dombrowski came over — signing Price, gutting the farm for Sale, Pomeranz and Kimbrel — have all shifted the roster, from rookie league to MLB, to a greater focus on this three-year window. That's happened, and we've debated it ad nauseam. It will mean, I think, an all but inevitable rebuild.
Having done that, there are two paths we can take from here. One is certainly the patience/flexibility path, but unless we get tremendous rebound years from a bunch of position players, that risks wasting another of our two remaining seasons with this rotation that we've expended so much to get.
So my reasoning is this: sign Martinez for six years. If our top three starting pitchers can stay close to healthy and a few of the position player core have good years at the plate, that could be a great team. Take two swings at the postseason in 2018-19. From there, we'll see. Maybe we try to reload in 2020, but if it doesn't work, we're sellers at the deadline. If you make good trades, you can be back in contention pretty quickly.