Mjolnir is an extremely powerful magical hammer, sure... it's still a piece of metal. Okay, it's durable enough that when it hits Thanos' body it doesn't crumple or break as if it is normal steel... but it still needs to be packing sufficient force if it is going to hurt Thanos.
Cap was producing sufficient force with Mjolnir to put Thanos on his backside, he was even giving it to him with Mjolnir and a frigging knee for crying out loud. To me, that's as much an indication his physicals were amped as him summoning lightning is an indication he now has control over magic. But even if we set that aside and say his physicals weren't amped, my point remains that sufficient force needs to be generated to affect Thanos in the first place, super durable metal hammer or not. Either way, Cap produced a sufficient amount of force.
Now... let's talk about speed and skill. Cap has, consistently, shown speed and skill well beyond the likes of Thor/Loki/Hulk... he's even shown better speed and skill (might as well just call it skill since in a real fight the two are almost synonymous) than the likes of Iron Man, Ultron and Thanos... except the problem has always been that even if for every 3-5 strikes he lands on one of these guys, none of them hurt, so the 1 strike they land in return actually hurts Cap and he ends up losing. We saw this with Loki, we saw it with Thanos, we saw it especially with Ultron.
Now if we think about Hulk... he's someone who gets outpaced, outboxed and outpunched by frigging Thor... the same Thor who has been consistently underwhelming in just about every hand-to-hand fight he's ever been in. He literally just acts as a tank most of the time. Ultron? Thanos? Kurse? Even if you look at his fights against Loki, Loki tends to land more consecutive blows early on in the fight, but Thor tends to just outmuscle and tank his way through to gain the edge (in fact he was outright saved in Avengers by the jet).
If you look at Cap's fight with Loki, he was dodging most of Loki's strikes, countering with his own (which would do nothing), and then Loki would respond with a strike that actually has an impact.
Except now, for whatever reason, Cap along with his magical lightning hammer, can generate enough force to put Thanos on his ass, a guy who pretty much shits on everyone else I mentioned in this post (Cap even outperformed Thor against Thanos... Thor barely landed a single strike and just got slammed into the ground and beaten... Cap landed, including lightning, 13 consecutive, unanswered strikes on Thanos)... and in terms of speed and skill Hulk is damn near at the bottom of that rung, noticeably beneath Thor... Cap being at the top of that rung and now with a weapon/powerset that can seriously hurt Hulk, and, grants Cap far more versatility like remotely or directly summoning lightning, lightning empowered strikes, ground based AoE lightning, throwing and summoning the hammer, using the shield to cancel all of Hulk's force or just throwing it as a distraction... I fail to see how Hulk doesn't get completely and utterly wasted here.
It's like someone said, this is basically Thor but far more skilled and fast... I doubt Cap is nearly as strong or durable as Thor but it hardly matters because for all intents and purposes he can hurt Hulk, and that's all he needs.
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