The selling point of a game is never what language it was written in. That is one of the last things (if it ever is) on a consumer's mind when deciding whether to spend money on a product. It mostly comes into play when considering modding.
Minecraft isn't written in "native code" either. Have you noticed any problems with this demo? I haven't.
> probably without offline support
Wrong.
> need them to be online to do multiplayer
? Of course you need to be online to do multiplayer...
> redownload the whole thing every time you clear your cache and want to play it again
What cache are you referring to? The normal browser cache won't affect this at all. Sure, you can remove appcache, localstorage, indexeddb data, but why would you? That's the equivalent of uninstalling a game.
> have to deal with browser quirks and the browser adding a level of instability
What does that have to do with you as a user? The developer has chosen to work around such issues because he/she sees benefit to being in the browser.
>? Of course you need to be online to do multiplayer... You know on many desktop games you can host servers without needing any central authority or even your internet online right? Like in Minecraft? Quake? Halo? is the LAN forgotten?