Players themselve should still have the option to turn that back on. Its not a must to do. You could play the game with the poor textures fine. And initialy it could at least have the map files with just low res textures and lightmaps.
It just prevents player from having to download all the 15GB in 1 go, instead it would be more like 5gb worth of data (remember, hats and such are optional content and can still give the full gameplay so they dont have to be preloaded).
The minimum for a playable game would be:
The engine
Player models
weapon models
hud/interfaces
maps (optionaly not even all of them, it could for example load the map when it detects/loads the next map)
props
low res textures for props and weapons
exceptions for gameplay affecting cosmetics (harpoon, gatebot hats)
Everything else can simply be downloaded while in the main menu, loading the next map. During setup in mvm/comp. And if it becomes a problem you can even turn that off, or when you have a fast enough pc, you keep it on.
And this content could all simply be set active on the loading of the game. This means that whatever was loaded while playing wont show until the next restart.
And i am aware some might be able to use that to get rid of the hats on players.. but i personaly dont realy care, most players wouldnt even bother of trying to get rid of them using such method.
And im also aware it currently would give issues in the engine, but that is again the reason why tf2 isnt going to get it unless a new game has something that can be ported to also work in tf2.