Iggloo
Fly, Eagles Fly
It would be insanely stupid for the Eagles to franchise tag Foles. Foles would sign the contract immediately. That would fully guarantee the contract. He'd hold a press conference saying he wanted to play for the Eagles for the rest of his career. He'd say if the Eagles traded him he'd refuse to show up for the new team during camp. That would nuke any 1 year trade talks. Any team trading would have to agree a long-term deal with him for a lot of guaranteed money AND pay the Eagles trade compensation. Not going to happen. Wentz will also want an extension which the Eagles couldn't afford so the Eagles would be forced into under-selling him. Then they'd have the problem of what to do about Foles in the future. They don't want a repeat of the Redskins - Cousins debacle. It would be a mess.
The Eagles already hold the winning hand. Foles has out-performed his contract, they've squeezed an extra $2M out of him, and they're likely to get a compensatory pick in 2020. They'd be insane to screw it up when there's so little potential upside to tagging Foles.
The issue here comes down to this: By tagging and trading Foles, they might get a pick this year, in the 2019 draft. By letting him go as a free agent, they would get a compensatory pick, but not until the 2020 draft.
It's all about getting the pick this year if possible. And keeping him from wearing Redskins burgundy and gold, or Giants blue.