I agree. DOM will come, thinking that GC can not be developed in language the browser (and javascript) is built in is a bit naive (not to mention GC is a complex solution for simple non-problem, I would rather use malloc/new and free/delete + destructors than relying on Terminator Skynet ripped AI doing this really simple task - tribute to java and inability to free memory when needed! Anyway with each tab running in separate process, system will take care about memory leaks ;) ) while the runtime - the last time I checked, the CDNs were serving javascript libraries over mb in size and I really see no problem for them serving python runtime, while for c++, the runtime can be the browser itself and even if you pull in libc or proxy to the existing browser functions, this can be really, really small. Believing that code, that was designed to be human readable (as js), can be shorter than the opcodes doesn't really fit here.
I have heard this runtime considerations before but they are taking into account that someone will run webassembly compiled JRE in browser (and I am sure that right now, there are some freaks trying to achieve that). But there is still good old C++ (or Rust) not as productive as scripting languages but this is just due to much less open source "technological stack" (or some might argue, tribute to low quality of software today, "technological garbage") rather of not beeing capable of beeing competitive.
I think that the coolest trick that web based technologies pulled out was to convince the world, that the whole GUI needs to be primitive, simplistic, as they were unable to create something that desktop applications were doing for decade (speed/size considerations) and it might just happen that we will return back to owner drawn controls, where you wont be able to make distinction if app is running locally or in browser (no, I don't mean Electron, pun intended). There js will be unable to compete.
Anyway, push for webassembly is not here as corporations want to do something good. They want to push all computer users into old mainframe scenario, where you would have to rent space and cpu in a cloud and have just dumb terminal and javascript has hit its limits to replace desktop environment as it is, even with Windows Metro look (another simplification of GUI, done for mainframe scenario, Windows 365 are not far away)
Javascript was usefull in era of simplistic web pages as a small hack into html, but for next step we need something more and here there are other languages that can offer much more but are now limited to running in backend as they don't have browser support. But this will change with webassembly.