This reminds me of the signing bonus loophole(s) that existed in the early and mid 90s. San Francisco started leveraging the prorated nature of the signing bonuses, allowing them to lure in free agents with big payouts but spreading the costs out over the length of the contracts. Jerry Jones (and others) argued that it was a violation of, at least, the spirit of the cap but the league gave the tactic it's blessing. That let the 49ers to be very aggressive in the 1994 offseason, allowing them to sign some big name players like Deion Sanders and even snatch Ken Norton, Jr., from the Cowboys (an effective 2-for-1 move since it negatively affected their biggest rival and primary obstacle to a championship).
The Cowboys said, "two can play at that game" and, during the next offseason, gave Deiom Sanders a $35 million contract. I believe $15 million was signing bonus which, according to the rules at the time, would be evenly spread out over the contract's give year length). The Cowboys returned to the Super Bowl after employing the tactic and beat the Steelers to get another Lombardi Trophy.
This "void years" issue seems similar. Perfectly legal according to cap rules, but probably shouldn't be. Or, like with the signing bonus issue of 30 years ago, should have a few more limits, rules, and/or stipulations attached. I don't really have an issue with how the Eagles are using the loophole, but that also doesn't mean that it shouldn't be closed. While it exists, every team should look to use it, the Cowboys included. But it probably would simplify things and, I suspect (though I'd have to give this a little more thought) it might provide an advantage to large market teams or those with crazy rich (even by NFL standards) owners.