What I think would be a reasonable solution to this, would be to update the Source SDK and have custom assets downloadable though that. All map assets including ones from stock maps are packed into their respective map, and all mapping tools are stripped from the base TF2 download.I think that's not too bad or too surprising. Putting the assets in the game benefits one group (mappers) while being a waste of filesize for everyone else. That filesize will add up quickly, especially if valve continue with their habit of focussing on custom maps with extra content and adding lots of maps. Its not that hard to just find and download the assets. Perhaps we could curate a list of what's in what maps?
Anyone wanting to simply download and play TF2 would be only downloading the core files and the now smaller maps.
Mappers on the other hand would have to download the Source SDK, but that would access to the assets. A bigger download for us, but we're the ones wanting to use them.
Also a throw-in suggestion that they give the SDK its own workshop so that people can upload their own props/textures/etc.
It would be one hell of an update to implement though so I doubt that it would happen.