The questions you ask are great, but slightly less important when you add the fact that everyone has the box already, and getting them to use a new box has been tried incessantly over the years, and yet here we are with the same old box.
Tl;dr -- pragmatism is why the web ecosystem is eating the world, not optimal design.
There are three players in the game right now all of whom can push updates to the vast majority of their user base very quickly. This isn't a problem like it was back in the day. Now it's pure stubbornness.
See also, Excel and VBA in corporate America. Will companies let employees install language environment X? Probably not, but they'll have access to Excel with VBA.
Tl;dr -- pragmatism is why the web ecosystem is eating the world, not optimal design.