Time will tell. For sure, there is a collusive/"be good soldiers" aspect to the way all of these teams refuse to exceed the tax. For them, it's a message to both the players (who refuse to agree to a hard cap but have one anyway so long as the owners do this) and a refusal to help out the revenue-receiving teams (who get half the tax money paid).
But that is to be expected, Middleton is not some newfangled rebel owner, he's been part of this ownership group for decades. And you don't hire someone named "MacPhail" to distance yourself from a Giles.
Nevertheless, it's defensible on baseball grounds. Break the tax threshold for a Rendon or a Bumgarner and you've got money issues again in a couple of years. Break it to acquire a Bryant or Kluber and you're still giving up talent in the trade. Break it for a Deline Betances or Wade Miley and are you really getting your money's worth compared to a cheaper younger, if less consistent, replacement? It makes sense to see if this team is worth spending big on in July, and if it is, there will also be plenty of money to spend next season. You don't necessarily want to borrow against the money you'll be saving next year when Arrietta, Gregorius and Roberston are gone right now.
I mean, the predicament the Phillies are in, if they are in a predicament, is that their three highest-paid players besides Harper (Segura, Cutch, Arrieta) may not be worth the money and are aging, declining or injured. Ditto their completely unavailable highest-paid reliever. So the fix for that isn't necessarily signing even more guys in that category - and they already needed to add two.