HTML is capable of a dynamic interface, but it must be statically defined. With HTML5 we even have videos. The linked webpage including video work like a charm with JavaScript disabled. Furthermore I'd welcome a HTML6 as long as it stays non-programmable, and don't see why it shouldn't be able to be a low-latency interface. The interface of the linked video feels very snappy.
I agree that HTML-only GMail in the browser right now is a mess, but that's because a browser is not an e-mail client, and shouldn't be one.
People are trained for certain behaviours and often don't want to change from that. On the other hand they like new shiney software and things to try out. That explains in my eyes why for example Google Docs (seriously, office editing in the browser?!) is used. Also, Google has a good reputation for software.
I think most people are simply not aware of the dangers, downsides and alternatives of this approach. The behaviour was learned when people talking about a globally snooping NSA were called crackpots.
On top of that, Google has no business interest in offering a standalone office solution, so people either use Google Docs in their browser, or not at all. Supply creates its own demands.
How discoverability and ease of obtaining trusted code is solved, is a technical problem. I could imagine markup-only webpages which can call locally installed programs to appear in the tab, but the prorams are installed in trusted ways. That's just one spontaneous idea.
Mozilla is making it easier for people to use technical solutions that are suboptimal for them. This has a network effect and shapes the web to be a worse place. That is what I accuse Mozilla of, and I stand by it.