Browsers will never run games that aren't toys or use very simple assets in a way that doesn't completely suck. High quality assets need gigabytes of data. You either require users to download all the assets upfront (the thing we're trying to avoid) or streaming the assets dynamically.

You end up having to re-implement steam to keep a local copy of the assets on the client device yourself, expect browsers to do the same to manage caching the gigabytes of data transparently, or design your game around a very slow storage device or use tiny assets.

Flash games worked because they fit very nicely into the 'tiny assets' category.

PWA to cache everything locally?

Streaming assets seems reasonable.