It would be like if DC didn't own Batman or Superman. Its unthinkable. Tbh I would have rather seen Marvel Studios go bankrupt than having to deal with all this back and forth rights ****. Wish Marvel never sold the license. Even if it did keep them a float. The MCU will never feel whole as long as Spidey isn't part of it. They can try all they want to fill the gap with other characters but it will never fill the hole left behind. Even with the rights sharing deal you could tell they were held back by what they wanted to do.
Strangely enough, an argument could be made that if Marvel had never sold the rights to their most popular characters then they would not be as successful as they are now. You compared Marvel not having Spider-Man to Warners not having Batman and Superman but you neglect to acknowledge the irony that Marvel managed to be more successful than DC(which has all their characters) just by working with characters who were far less known. Case in point: we now live in a world where the first ever Justice League film with Batman and Superman, and Wonder Woman failed to out-gross two movies about a tree and a talking raccoon & movie about Black Panther is, as of now, the most successful solo superhero film of all time ahead of movies about the A-list juggernauts like Superman and Batman.
Marvel was able to keep the superhero genre fresh by introducing audiences to new characters like Guardians and BP that have become more popular than even their one time more popular Marvel brethern like X-Men and Fantastic Four. Black Panther alone still made more money than any Spider-Man and X-Men film ever did. Unlike Warners, which they always had Bats and Superman to fall back on; Marvel, in contrast, never had that safety net, which then required them to work harder on making the best movies out of their lesser known characters. As they say, "Necessity is the mother of invention,” so Marvel had to work with the leftover scraps and turn it into gold -- which turned out to be the best case scenario, Marvel gave us movies about Doctor Strange, Avengers, and Gaurdians -- shining the spotlight on their B or C list characters and diversifying the genre. Frankly, this makes me thankful Marvel didn’t have access to their big characters or else all they would be making is X-Men & Spider-Man movies akin to how pre-MCU the only superhero things that Warners had any fortitude to make was pretty much Batman and Superman movies and nothing else. As a DC fan, I have to thank Marvel because if it wasn't for them then WW, Aquaman, and Shazam movies probably would never get made, and Warners never would have learned that all so valuable lesson that you can indeed have DC characters not named Batman and Superman be successful. In fact, I'm so envious of Marvel's success that I almost wish DC had done the same thing and licensed their A-list characters to various studios so they can follow a similar trajectory to Marvel that lead to the creation of their own film division.
Not to mention if Feige didn’t gain the experience from working on the pre-MCU Marvel films like the early X-Men and Spider-Man films then Marvel Studios wouldn’t be so consistent in quality that they are now. Feige learned from the mistakes made on those pre-MCU previous movies and applied the lessons learned from his experience making those movies to the MCU movies. This is a large reason why Marvel continues to have an impressive streak of quality that no other studio has yet to match.
Face it: Marvel selling off their big properties to other studios was one of the best things to ever happen in the genre. What appeared to be a disadvantage turned out to be a blessing in disguise. Marvel would never even bother to have made a Guardians film if they had characters like Wolverine from the start. By the way, the success Sony had with Spider-Man along with Fox's success with the X-Men franchise is what prompted Marvel to start making films in the first place. So while I'm bummed Spidey won't be able to tag along with his fellow Marvel brethern(for now anyway) this won't make me wish that Spider-Man was never sold off because it really benefited Marvel in the long run as well as the entire genre(Spider-Man 2 & Into the Spider-Verse also would've never have been made had Spidey not been sold off).