They're not really robbing the future, as the value of the franchise will rise and the income will rise. There was a good article on ESPN yesterday discussing how Bruce Sutter, who retired 30 years ago, is still getting paid. Bobby Bonilla is also a famous case, but the Nats' deferred deals aren't like those. They basically create an annuity for the player, and the player can decide if the net present value is worth it or not.
I think their problem is that they try it with all of the large contracts, instead of really feeling out if the player is up for it or not. Regardless, it's normal for the Rendon and Harpers and Zimmermanns and Desmonds to leave via free agency. Rendon wanted more money now, and walked.
On a related note, one less Boras client on the roster isn't a bad thing. The Nats now have budget for 2-3 key pieces. Rendon is a good player but he's not a HoF guy, and probably not worth that amount of money. Sign a solid free agent for 3rd base and sign some decent relievers who aren't retreads or high-risk options.