(which shows up in X-Men evolution and which I sometimes see discussed for fics or roleplaying game & so on)
First, Erik was in Auschwitz - in comics canon, from as a early as Claremont - Auschwitz was liberated by the Russian army, not the American.
Given that there’s a trend in some American fictions to erase various Allies contributions to the war in order to recast them as American victories, there’s something revisionist about making it Captain America that I don’t like.
Second, Erik’s backstory during the Shoah was explored in a very good mini called Magneto Testament (and if you want to write pertaining to that topic you should read it because it’s very good in term of both portraying the Shoah, and showing how it was formative to Erik in very subtle ways as well as the obvious stuff. Very good, nuanced characterisation); in that mini the author (Greg Pak, one of the few comics PoC writer btw) emphasizes that Jews weren’t only victims as the narrative very often cast them as. That they did fight back against the Nazis. Erik’s -or rather Max’s at the time - uncle Erich (yeah) takes part in the Varsaw Ghetto uprising. And when Max and Magda leaves Auschwitz, it’s in the context of taking part in a sabotaging effort and then running away (no death marches for them, and very shortly after all of the Romani in Auschwitz were killed off - how Madga was rescued from that is very gruesome).
Now, I like Steve Rogers and I mostly like “Captain America” (in great part due to reading the Adventures of Kavalier and Clay) as a symbol of everything that is good and hopeful about the US. But it’s very easy to twist that in a fashion that is problematic and to fall down into what I’d call “white savior Captain America”. To me, recasting a Max and Magda who ran away from the Death camp with their own resources and the help of other fellow prisoners is doing exactly that.
Third, from a narrative point of view, I think it’s also very important that Magneto was not saved. This is the point of Magneto. There were no Righteous Amongst the Nations for him. Magneto’s narrative is that the world did nothing while the Jews and the Romani, the disabled and the homosexuals, the resistants and the communists, and all the others were slaughtered by millions. If there was, he wouldn’t have become Magneto.
So it would be using Captain America’s narrative of being a WW2 hero and make it a priority over Magneto’s narrative. IOW, making a white American character’s narrative take precedence over a Jewish European’s narrative.
It also could build a narrative of the US taking part in the war in order to rescue the Nazi victims, or doing all that they can in order to do that that is, at the very least, a topic of debate.