The way the class has come to them I think they should make a move, though it still seems likely they greatly prefer Arrieta and don't rate the others as highly. But next year's class isn't any better or worse. Which is kind of the point of their approach, really - they don't think free agency is a good way to acquire a top starter. You either develop, trade for an established young arm, or trade for an ace's last few years.
So yeah, I don't believe free agency is the way the Phillies get their 2019 rotation. They could add as many as two more arms from trades (one in July, one in the off-season) that are both better, younger and no less controllable than one of this year's free agents on a three-year deal (albeit, you don't get them for this year). If they do sign one, they'll still be trading for at least one. Of course if all these internal options completely fail then they'll have to look at FA for a 4th starter type too.
One of the phrases in that 538 article referenced it being worth it to get better even as your farm system declines. So that obviously also means trades, as well as giving up comp. picks and not drafting as high.
This stuff doesn't exist in a vacuum. There are certain pitchers they want and certain pitchers they don't want. The pitchers they most want to trade for haven't come available yet because I believe if they had they would have done it (the pitchers who were traded weren't guys they wanted badly, no other team has met the price for the others). It seems clear they want Arrieta, and equally clear that they only want Lynn or Arrieta on a really friendly deal (like, possibly Moustakas-friendly at this point).
At the same time, if Arrieta's the guy it's also possible you can't write him into the 2019 rotation.
The advantage of Keuchel or Hamels over Arrieta is you're signing one of them in 2019 instead of 2018, plain and simple. If you're not signing them to 4-5 year deals - which seems like a bad idea - or these guys are not going to be good pitchers in 2020 the specifics of the idea that they should do it now kind of evaporates.
I gotta think Hamels A) won't be on the Rangers by August and B) will have his option picked up. Partly because it will be a different team, But also because his buyout is $6 million against $20 million. So you're only really spending another $14 million. But who knows what kind of year he's going to have? He'll be pitching at 35 in 2019 (which kind of makes his whole objection to the six-man rotation funny. OTOH, routine is important for pitcher health. OTOH, maybe the new-school stuff could extend his career by two seasons, instead of being a "horse" who comes up lame?).