Having the OS, the hardware, and the app store, along with a lot of the (almost mandatory) cloud services AND the most popular apps, all made by the same company.
For a smartphone, I can pick between 2-3 (depending how you count) major OSes, tons of different hardware vendors for the non-Apple ones, one app store if it's Apple or any source if it's not, whichever cloud any third-party app uses or no cloud if I desire, and a lot of popular third-party alternatives to the native apps.
All that choice is already there, just for a phone. Previously you'd choose a flip-phone and have it all locked together, including with the carrier.
I don't think I would describe Apple as a monopoly. But I would say that copyright and patent law helps tech companies, like Apple, by restraining competition.
All I know is that I have it disabled on my phone except for the couple of services I do want (like Find My), and it works fine. My photos get backed up using Google Photos app, etc
I think if you turn it off entirely, it'll ask you once for each OS update if you want to sign in, which isn't too bad.
When it really screams is if you log into iCloud then forget your password and it keeps asking you to input it. Like with every elderly member of my extended family.
All that choice is already there, just for a phone. Previously you'd choose a flip-phone and have it all locked together, including with the carrier.