There's no "What if" to this, we're seen periodic attempts at making classic pulp hero movies all the time, what has invariably happened is that they turn out to be flops as they lack the major things that made the Marvel series a success.
First, you have to understand why the Marvel movies were a success. I know that folks like to say that the Marvel moves took a bunch of "unknowns" and made them huge, but the fact is almost all the Marvel heroes used had shown up in media. The 90s saw an Iron Man cartoon, Captain America showing up in the X-man series, and, of course, years of active comics as well as many video games. While they were certainly not AS big as Superman, Batman, and Spider-Man, they were not entirely unknown, and the idea they existed in a single setting was a long established idea in many people's minds. This meant that those characters had pre-built fanbases who would form the core of your get people into seats efforts and those people ALSO tended to be more involved online and the internet and social media served to amplify their effect.
Secondly, you had decades of experimentation in comics to know what worked and what didn't in storytelling with those characters. This meant that the adapters could pick what BEST worked for those characters and the dynamics and symbols they represented. This meant there was, in some ways, less risk involved.
Finally, you had to have the timing right. The Marvel Cinematic Universe actually started picking up steam when people were getting TIRED of traditional superhero movies and were ready to try something new and inventive with them. Without the priming effect of the late 90s and early 00s superhero movies to set the stage and get people fatigued of the traditional superhero formula, the MCU likely wouldn't have done so well with it's new formula that amounts to "X movie genre WITH superheroes" (seriously, almost EVERY Marvel movie falls into this structure, with the notable exceptions either being kinda lousy movies (Captain Marvel) or the Guardians of the Galaxy).
You don't have ANY of these effects for pulp heroes. You have nowhere near the breadth of material to draw on to know what works or what doesn't. Most of the characters do not have the longstanding existent fanbases, and many of the biggest fans of the old pulp heroes tend to older generations that are not as active online.
Finally, you also have the lack of blockbuster pulp hero movies to show that such movies CAN be successful. The MCU would never have happened if the Raimi Spider-Man trilogy or the X-Men movies had done financially poorly. In point of fact, executives would look at prior pulp hero movies and be, well, very concerned, as in the 80s and 90s you actually saw quite a few pulp hero movies made and they tended to do... poorly.
The Shadow only made $48 million while having a production cost of $40,
The Phantom outright LOST a considerable amount of money.
The Rocketeer only so profits roughly on par with The Shadow. While the original Conan the Barbarian movie was a major box office success (making nearly 10x its cost), the sequel only made money comparable to the later pulp hero movies. In the late 90s
The Mask of Zorro did quite well too, but that was mostly on the star power, and its
sequel saw a drop off, rather than improved returns despite having the same stars.
Long story short, even if one production house somehow managed to get all the rights to these characters under the same roof (a hugely complicated process), they would be unlikely to risk such a crazy idea as the MCU with them, as there is considerable prior history showing that these were not franchises people were interested in.
