One think that makes Apple's iOS a monopoly is the lock-in of assets.
If you buy music, movies, books, ... through your iPhone, you won't be able to transfert all that ownership to a Kindle, an Android or some other (non-Apple) device...
You also have a lock-in through the hardware : iPhone chargers don't use the standard USB connector, many hardware peripherals are locked to the iPhone through that lightning port, AirPods and other can't realisticly be used using a non-Apple hardware, ...
This also extends to your iMessage contents and some other...
All these lock-ins make the Apple ecosysteme it's own environment (market)... and in that market, Apple has a strict monopoly... The argument that the people may switch to Android is moot as it's a different environment where the assets won't be available/usable.
On the other hand, last time I checked, there are no film, music or books shops on the playstation. There are no carkit, soundstation, tracking devices, headphones, ... specific to the playstation. The only assets linked are the games and the VR system (the controllers can be connected and paired to a PC and used through Steam for example, you can use the PS4 Camera on a PC). That's why you see so many people hopping from one console to the other.