Yes, you lose many of those capabilities, but you also gain new ones. I bet agar.io would never have been played had it been a Windows executable or a Steam gam

Yes, you lose many of those capabilities, but you also gain new ones. I bet agar.io would never have been played had it been a Windows executable or a Steam game. The friction in starting the game and playing the game is so low, that it drawfs all other downsides by far. In fact, this is the exact justification why HTML/JS business apps have thrived, because even though JS has tons of problems, all of them can be overlooked when you gain the capabilities of the web.