I feel like all we need is "one Ring to rule them all" - one app to run on all the mobile and desktop OSes and in web browsers and I can see no reason why do we

I feel like all we need is "one Ring to rule them all" - one app to run on all the mobile and desktop OSes and in web browsers and I can see no reason why do we need different kinds of apps for these (there hardly even is such a thing as a web site today actually - all the websites, this included, are apps, just styled and implemented differently and optimized for bigger screens). Perhaps Flutter (or something else to come) is going to let us achieve this.