The other Marvel Superman stand-in's definitely bring up a good point; Marvel has tried to introduce their own Superman several times, and it doesn't ever really work. So would using the original version be successful where the proxy attempts have failed? I don't know if it would or not, honestly. I think those other guys are a cautionary tale for any other publisher looking to use the character, but guys like Blue Marvel, Hyperion, or Sentry....they might be Superman-like, but they're not Superman. Public domain Superman means you can use the name, Clark Kent as a mild mannered reporter, and Lois Lane. There's a difference between a stand-in and the real, original deal....but I don't know if it's a big enough difference to warrant a different result here. Maybe the Superman archetype just doesn't fit into the MU, or maybe the niche Clark occupied is already taken and he has nowhere to go, or maybe those proxy characters failed because they were proxies and Clark would be more successful.
And it's not like Marvel needs the character either, they've done just fine without Superman up to this point.
But I think one thing that might make a difference is the power level. We're talking Action Comics #1 here; the guy couldn't fly, didn't have super senses or heat vision, and wasn't half as strong, fast, or durable as he'd eventually become. As far as power levels go, he wouldn't necessarily be in the same ring as the heavy hitters like Thor, but in the mid-range like Spidey. That shift in scale might make a difference in how well Superman fits into the MU framework. Or it might not, I honestly don't know.
About all I do know for certain is that if Marvel got their hands on public domain, Golden Age Superman, the next Superman movie would be a hell of a lot better.