> Why is it so complex to have a foss mobile OS.

This is not too hard. What is hard is to trust it enough. A FOSS OS, by definition, allows to install whatever software, and allows for modification of itself. It is built to overcome limitations, not impose them. In this regard, it's a perfect tool for a criminal who wants to circumvent security measures, because these are limitations. It's the same problem as with cheaters in online games, only with more than games on stake. Banks and payment systems want guarantees of integrity and protection, including protection from user's actions.

A FOSS OS also assumes that the user values the freedom, and is competent in its technical aspects. This is emphatically not true about many users. They choose iOS because it's locked down and thus they cannot inadvertently do something they don't understand, and can't be bothered to learn. More importantly, their grandmother cannot do something she doesn't understand but scammers persuade her to do.

It's a bit like driving on public roads. If you want to drive yourself, you have to reveal your identity and obtain a license. If you want the hassle, take a bus, but buses only go along their routes. Letting unlicensed people drive cars where they see fit was found unacceptably dangerous for everyone eround. Maybe mainstream mobile software development will follow this model, too :(

> It is built to overcome limitations, not impose them. In this regard, it's a perfect tool for a criminal who wants to circumvent security measures, because these are limitations.

None of those limitations actually provide any security.

In order to use your bank's mobile app, you need your bank login credentials. It does not matter how secure a bank app on your phone is or whether it requires some kind of attestation because the attacker is going to get the victim to type them into a fake app or the attacker's web page which don't require any such thing and aren't even necessarily on the same device. And then it does not matter what kind of device you require the bank app to be installed on, because the attacker will get one of those and use the phished credentials in it.

There is no security value in requiring things that are useless.

> A FOSS OS also assumes that the user values the freedom, and is competent in its technical aspects.

This is not an assumption at all. The user is not required to write their own software or install anything from outside of a trusted repository. The value of the OS to such people is that someone else can write that software, and then as it matures it makes its way into the trusted repository.

But if mere mortals can't do that, if kids need an ID and a credit card in order to learn and experiment and hobbyists hit friction and spend their time on something else, then those things are killed in the cradle and never exist to begin with. And then instead of free software made by the people who wanted to use it, you're left with only apps made by predatory for-profit corporations and scammers that make it into the official store because their scams are profitable.

> It's a bit like driving on public roads. If you want to drive yourself, you have to reveal your identity and obtain a license.

It isn't a public road, it's your own phone.

> They choose iOS because it's locked down and thus they cannot inadvertently do something they don't understand, and can't be bothered to learn. More importantly, their grandmother cannot do something she doesn't understand but scammers persuade her to do.

For what it's worth scammers have zero problems scamming grandmothers with Apple computers and iphones.

All this is true about Linux on desktop, though my bank still allows me to log in to online banking.

At least for now.

I'm not aware of any major issues this has caused.

The trust isn't the issue. Google and Apple has made DRM easy for these companies to integrate, and therefore they do it. There isn't more to it than that.