Planeswalkers killed Magic, everything else was just icing on a shit cake.
I think they are a symptom of what killed Magic.
What really fucked over Magic, especially the Vorthos side, was Hasbro chasing that Hollywood dragon. After the Transformers movies, Hasbro for desperate to license any and everything they had out. That's how we got outlandish exceptional premises like Stretch Armstrong movie or The Monopoly movie that had Ridley Scott attached IIRC.
Maro has heavily implied if not it right stated, nerfing the Planeswalkers was to sell Magic to Hollywood. He puts it in typical Maro fashion of "Corporate does nothing wrong" by saying you can't tell stories when your characters are as powerful as premending Walkers. Of course you can, but when you're making a mass marketed venture, your gotta account for the lowest common denominator.
I believe it was also stated that's why they were made into cards, to make players have a closer attachment to them by actually getting to play with them.
As far as I know, it hasn't been stated, but I can infer that this mindset was the driving force of the Gatewatch arc. That was the movie cast and there big budget trilogy arc. Introduce them vs the Eldrazi, refer to Bolas. Amonkhet as the Empire Strikes Back with Bolas beating them. War of the Spark finishes it off. Probably the hope of some spin off movies in-between as cinematic universes were the best way you could get a big budget movie at that time.
That's why Gigachad Garruk was shelved for a chick, and Nissa's edge was filed off from Elf Hitler to literal tree hugger. Probably same with Ajani and Gideon, as the big cat man probably is expensive for a Main Character and not Sidekick like Chewie.
However, what really is killing the game is their new CEO from 2018(?). Hasbro put him there for one purpose, to wring every bit of cash out of Magic as he could. And it's worked, 75% of Hasbro's profits last year came from Wizards of the Coast. However, I think that's going to be a short term gain that'll drive the franchise into the ground.