> If web dominance changes I'm still left with an application that works everywhere.
That's the faulty assumption in my view. It doesn't work everywhere - even on the web, you're bound to whatever the dominant browser thinks is right for you.
Some random examples from recent months - 3rd party cookies (putting aside the fact that they were enschitified for ads), Google suddenly decides to turn them off and they're gone. Ad blockers don't make it as an extension (so extensions are not, in fact, a solid substitute for apps). See AMP - you don't even get the choice if your website will be transformed into whatever Big G thought was an "optimised experience". What's to stop them from forcing you to enable "privacy-focused topics" or Web Bluetooth to sign into your Gmail account? Let's not even begin to discuss Search - all from your browser company.
You see, browsers have become much more than simply web agents. Your app's documents and hypermedia are technically downloadable "everywhere". However, just like on an App Store device, if and how users can discover your web content and how they experience it is entirely locked inside the walled garden of your Chrome-like browser vendor.
PS: Since before the browser Interop initiatives (now more than 3 years ago), Safari has been leading the race when it comes to accepted standards.