Nadal's speed and defensive skills/passing shots rattled Fed big time but it's not the only reason he has had so much success against him. On many key points in their matches he was the aggressor because Fed's BH couldn't take the barrage and buckled under Nadal's heavy lefty FH.
I think you nailed it right there. That is the SINGLE BIGGEST reason why the Nadal-Fed matchup has turned on it's head.
Fed could never be aggressive on a ball high to his backhand pre 2017. That was one weakness in his game. And Nadal could exploit it more than anyone on tour.
Again their matchup has evolved, and it's NOT like Nadal always hits 100% to Fed's backhand.
Still whenever Nadal used to hit HIGH to Fed's backhand, he could: Get into offensive on a neutral rally OR get back to neutral terms when he was behind in a rally. Meaning Nadal always had an OUT when the pressure was high.
Not anymore. Fed can attack with his CC backhand and stretch Rafa out wide on the Spaniard's forehand (where Rafa is weak).
Also the reason Fed's backhand has hurt Nadal the most (vs the tour) is because Fed's backhand previously was still good on lower and flatter balls that the rest of the ATP hits. (Sure he's gotten better at that too. But the improvement is marginal.)
The biggest difference is how Fed can step inside the court and take up Nadal's spinning ball on the rise and hit through it with his backhand. He's waaay more aggressive there and doesn't run around his forehand as much.
Also with age, Nadal is much slower especially laterally, so that hurts Rafa. He is also MUCH WEAKER with defense on the forehand side when pushed wide and his ROS is just garbage these days. (I'm always struck by how poorly he returns Fed's serve now, unlike the past). Those factors also contribute to Nadal losing to Fed.
But the main reason still is the CC FH to CC backhand exchange.
I honestly feel that unless Nadal improves some things significantly he won't beat Fed (in a big slam match) outside of clay. He can hope for Fed to get old, but given how good the Swiss is at 38 - that's like hoping against hope.