[flimsypremise]

In web application terms, the "build" is everything that needs to happen to get your application running into production. That means a runtime and dependencies. Speaking of dependencies, does your perfect frontend simply not have any of them? Is every tool you will need to use perfectly packaged with vanilla CSS and ES6 modules? Browser support for import maps is around, but its nothing I would build a production application on. And god help if you if you work in a context that requires support for older browsers.

Maybe in 5 years this will be a practical approach, but there's a reason that old ways of doing thing hang around: they're well-documented and reliable.

[coddle-hark]

I mean, people ARE doing it, and like I said it's mature enough to be the default way to build Rails apps. There's tradeoffs, no doubt, but this is absolutely a valid, productive way to write (certain types of) web apps.