Earlier quoted context omitted.
ETags still require a round trip. You could cache for longer but now you have to deal with the complexities and struggles of caching.
With HTTP/2 multiplexing all of those requests can be made in a batch without round trips. And complexity of caching? An Etag done right is content-based. There's no logic to worry about. It's really unfortunate that this style of architecture lost the battle. It's elegant. Data cleanly separated from presentation, small digestible entities, and it all kind of makes sense. But what killed it was the verbosity of XML,…
Hm - how would multiplexing help here? Does the browser read ahead and process cached assets to find dependencies before firing off etag requests in a batch? I'd be surprised if that was the case.