Users were perfectly happy with this and the DMA seeks a solution for something that wasn’t a problem... otherwise, iOS market share would’ve plummeted ages ago.
The problem is the cut that smartphone platform owners want to take from subscriptions to services, if purchased via apps. Simply due to the fact the app was downloaded from the only store allowed on the platform.
Services like Spotify, that compete directly with those of the platform owner (e.g. Apple Music or YouTube Music) have two unpleasant options. Either they give their customers the convenience of paying for the service through the app, in which case the platform owner takes a cut (15-30%, depending on platform). Or their customer needs to leave the app and subscribe via the service's website - all without the service being allowed to even
hint that this is possible. Even if they do so, adding friction to a sale will naturally result in less customers. Either way, the platform owner gets an unfair advantage for
its service.
If you want to argue this isn't
much of an advantage, you might want to ask yourself why platform owners are so insistent on maintaining the status quo. If most customers know how to sign up to services directly - and can be bothered to do so - then this policy wouldn't generate much revenue for the platform owner. In that case, why pointlessly inconvenience the platform's customers? The truth is, it generates a
ton of revenue - and as smartphone lifecycles have lengthened, Services has become an increasingly significant piece of the pie.
In the early days of the app stores, Apple and Google's positions were more defensible. They were putting a lot of effort into building their platforms, and deserved to reap the rewards. And they did - they're both among the richest companies in the world. But as time goes on, and smartphones have become less a luxury and more a fixture of modern life, it's worth revisiting the situation. Allowing these companies to have the sole app stores on their respective platforms essentially gives them a license to print money.
Google
happens to be more open than Apple, because their business is selling advertising space, and Android largely exists to stop Apple having a total monopoly on the smartphone market (and hence mobile browsing). If Google had left it to the likes of Samsung, Nokia or Microsoft to create alternative smartphone platform(s) to Apple, it would have been game over 15 years ago. So they developed and gave away an OS for free, to anyone who wanted it, and didn't object to manufacturers creating their own app stores. Google still tries to deter users from using alternatives to the Play Store, they just have a (much) weaker hand there than Apple.
In terms of iOS, the problem tends to be 'invisible' to the consumer, in that the unfairness most directly affects the companies trying to compete with Apple on Apple's platform. The result for the consumer of weaker competition is higher fees and less innovation than there otherwise would be. Without the counterfactual of a more open iOS, though, it's hard to know what you're missing. The iOS platform matters, not just because it's our chosen platform, but because at 2/3 of mobile app revenue, and 3/4 of subscription revenue, it's by far the most important mobile marketplace.