"To me the problem is right here: why must every web app work on all devices?" Because users want them to. If your web app doesn't work on their device, they'r

"To me the problem is right here: why must every web app work on all devices?" Because users want them to. If your web app doesn't work on their device, they're going to leave and use something else. The days of people being tied to their laptops is over. "And why, if you build an iOS app do you have to build one for Android too?" You don't have to, but if you want success, it's probably best to try and reach as many people as possible. And there are more Android users than iOS users.