There are several problems with the old interface, not the least of which is the simple issue of it requiring over a thousand not-quite-similar files to be created and distributed, with a lot of hardcoded constraints. Like, for example, only being able to offer interface graphics either in 1024x768, or 640x480.
The problem with HTML5 and other interface engines is that they are massive libraries we would have to incorporate, and there's a lot of glue code that would have to be written in order to make it functional. So, while a more flexible interface is certainly something we would really like to have, the amount of work required to do it is prohibitive.
If a rewrite happens, it is much more likely that we'll stick with things we already know how to do in the code, like adding more extensive lua scripting support or adding flexibility through better tables.
It's worth consideration, in the mix of possibilities. But at present I think we are pretty happy working and developing the in game UI as is.
We do utilise HTML5-esque features in the new Standalone server WebUI. It could very well replace the server on all platforms in the coming months.
I would argue that those two are not at all comparable. Generating HTML and parsing and displaying it are two rather different things.