If you can get Yelich (or even Stanton) and a #2 pitcher in whatever flurry of moves you make then of course you can afford to trade one of the OFs.
The argument in the article makes sense too really, except for the part where Williams probably isn't valuable enough to be the centerpiece of a deal. But you could easily get by in 2018 with Quinn and a free agent before signing a star OF in 2019.
I guess I just don't believe that Herrerra-Williams-Altherr is the 2019 OF even if they have their best possible 2018 seasons. I'm still not positive they are the 2018 OF, per se (Williams might be more like the 4th OF but will get a full season of ABs as a lefty). That doesn't mean they can't both be All-Stars at some point but if Altherr gets hurt again or Williams is still finding his way next year and the Phillies are ready to win (which they certainly intend to be) they'll upgrade for 2019, so with that in mind, anything is possible, even a year early.