The Marvel Cinematic Universe is easily the most complex and expansive universe in cinema history. Spanning 27 movies, 18 television shows, and still counting with more to come in the years following. For many of the MCU fans, it isn't very difficult to follow along with the canon as either they grew up with the MCU starting with Iron Man in 2008 or have been able to catch up to where it is at now.
But now with multiverses at play, former non-MCU Marvel films are now being reintroduced and put into the canon and even rumored to get sequels of their own, with Tobey Maguire and Andrew Garfield rumored to be linked to more Spider-Man appearances, what was already large could soon be massive.
This could take something that is already pretty complex and make it almost impossible for new fans to join in, especially with new movies and shows coming out on a regular basis. There could become a problem of continuity way too large for audiences to follow and not know where to jump in, or not feeling comfortable jumping in at a later point. Most audiences don't want to have to do homework just so they can fully enjoy a movie, they want to enjoy it as it is at that moment.
This same problem has occurred in comic books for both Marvel and DC, with both having decades worth of lore and continuity that fans would have to follow to understand the full story of what is happening. Now both Marvel and DC in their comic books have had different ways of dealing with that with DC changing and rebooting timelines like in Crisis on Infinite Earths and Marvel just creating new universes like their Ultimate universe. They both saw a problem for new fans and "fixed it"-although Marvel mostly builds on one large story still making one long story that is confusing even for die-hard fans at times and DC constantly has been running into the issue, therefore, rebooting a couple of times this past decade alone with the likes of the New 52.
In the future, the MCU could implement a reboot of sorts to create a more welcoming universe for new watchers. Since nothing like the MCU has ever existed in movies, this would be new territory for all and could be a gamble. Whether it would mean the introduction and exploration of a new universe, completely redoing the origins of the heroes we've already seen, or even scrapping all of it and just going back to separate universes like the earliest X-Men and Spider-Man titles, the MCU would have to implement something to make it easier to understand and take in for new audiences. Maybe something along the lines of the Ultimate universe would work best for the cinematic universe, with new takes on the origins and stories of everyone's favorite heroes. Or even just introducing alternate versions of the heroes like Miles Morales in place of Peter Parker as they have mostly the same origin but are different enough to both be compelling in their own stories.
The constantly growing universe wouldn't just run into an issue with bringing in new viewers however, another issue the MCU could run into is the matter of scale and impact of the moment. The MCU is home to some of the biggest movies in cinematic history with a massive culmination of over a decade of work like Avengers: Infinity War and Endgame, as well as massive crossover events decades in the making like Spider-Man: No Way Home. Although they are super exciting and fun when they come out, they also set a very high bar for future installments. Because that bar is so high, it could become near impossible for future movies to not meet that bar and not have the excitement on the scale that those other movies had.
When those films were released, audiences walked out feeling that they watched the pinnacle of cinema, and that's great, but that's also a problem for the future. Especially when it comes to the feeling that audiences get when seeing the Avengers return and assemble in Endgame and when the three Spider-Men swing together for the first time, those both created amazing moments and impacts on the audiences. But how does it go up from there?
That's why a reboot would be beneficial for the future of the MCU. They can reset the scale of the universe restarting something that isn't just a massive epic battle. That in turn can make the impacts of the moments matter again as when they do happen, it's fresh again and that "awe" factor is back when it had been lost. Also, if they were to reboot the entire universe the MCU could have a new set of standards much like killing characters off more frequently or even creating a new roster influencing how the universe differs from the previous. Either way, the MCU will need a reboot somewhere in the future.