This really isn't that complicated re: Eagles demise.
The greatest QB in the history of the game, Tom Brady, is nevertheless ineffective when he's under intense pass rush. So... if protection is struggling (due to injured O-line players or underdeveloped and unseasoned players, etc) then the value of a good running game helps alleviate that pressure!
Otherwise, what's left? Dink-and-dunk, and even against the so-so defenses they're usually 3-and-out. This is precisely why Belichick did everything within his power to make sure he had a decent O-line giving Brady time. And even at that, key pass rush defenders get injured from time to time, which is why having a good running back matters.
When Brady's O-line struggles and there's not much of a running game - he's just another pocket QB who struggles. But you put skilled running backs behind him like Sony Michel (NE) and Leonard Fournette (TB) - that keeps defenses honest, which buys Brady the time he needs to be effective. This is just basic fundamental football.
Carson Wentz is very similar to Brady in that he too is a pocket quarterback with a strong arm. The difference however is that Brady is much more accurate, especially in clutch situations, and Wentz holds onto the ball much too long. It doesn't help obviously when Carson's not working with skilled receivers who can create separation. Then you throw in that pedestrian lawn-chair O-line that struggled to give him more than 2-3 seconds to get rid of the ball, and a head coach who would rather catch COVID than call running plays...
That's going to have your pocket quarterback running for his life 3/4 of the game, and that's a great recipe for a 6-10 record with no postseason.
Jalen isn't a pocket QB. He's quick and instinctive. There were only a few plays last season in the games he played where he was indecisive and held on to the ball too long. Yet he too is going to struggle if he has no open receivers! And that goes back to the fundamentals. No pass protection, which means he has to get rid of the football quickly, which ain't happening if you don't have receivers who can burst and create separation within the first 5-6 strides of their routes off the line.
At best, Jalen will be a Lamar Jackson type QB. And then, obviously, they're running the risk of him trying to squeeze a few more yards out of a play than he should, which opens him up to getting hurt.
All because of a lack of running game, and very little pocket protection.
This was essentially the downfall for Carson at PHI. He was under pressure so much that he was forced to create something out of nothing. Then the slide began for him. His QB mechanics started deteriorating, he was having to get rid of the ball no sooner than he dropped back, which led to errant throws, horrible timing with his receivers, and then even on the rare occasion when his protection gave him time - he got lost in his progressions.
The only way Carson can be good again is if he has a coach who understands the game. Doug Pederson did not understand the game. He made his QB conform to HIS style of offense, which was one-dimensional and subsequently very predictable.
Who knows how Jalen will fare. I saw enough in the games he played to see potential. But that potential will never materialize if they continue signing old, slow, oft-injured players and rush once for every 5 throws.