The problem with a monopoly is it leads to market dominance which results in degraded customer experience, degraded quality and potentially price increases. There's a few major monopolies that really exercise this point.

With Google Search they're basically the only player out there. Sites are actually programmed now so that only Google and Bing can crawl them (as in search through their content to optimize it for being searched). This prevents are competitors from coming in, but it also means that they're going to be free to cram in as many ads as they can and change the algorithms to focus on sales rather than what the user wants to find. The result is that Google Search is now worse today than it was 10 years ago.

Similarly you have Ticketmaster. It's owned by Live Nation who also own venues, venue rights, talent agencies that organize tours... and also the scalping website. Over the last 50 years Ticketmaster has effectively wiped out all possible competition and locked out competitors from acts that they host. The result of this is that the largest ticket seller in the world can't keep their website online when the largest music star wants to tour.

You don't want 30 app stores on your phone, tablet or computer. But you want THE OPTION. I have like 10 reward programs on my phone, I choose them.... because I use them. But when my local autodealer told me that I was REQUIRED to use their app to even use their service, I just told them sorry not interested.

Epic's lawsuit isn't about forcing you to download their store on your phone, it's about allowing you to have the choice. If you want to play their games on your phone, you can.

Edit: And you already are sort of starting to get a degraded experience on Android and Apple stores as they allow companies to pay them for top spots.