Seriously... do we really need to invent yet another implementation of Python that runs on the web in Javascript? I mean it with heart. How about we just have one language and one implementation forget about it? Wouldn't that be super ideal?
Imagine you're a scientist, and you have a large, pure python codebase - let's say for intelligently predicting the number of foos in a bar - and you want to make that available via the browser, in order to visualize the progress of your foo-hunter.
This makes it a trivial task - and means you can use cool web-based visualization tools (D3, etc), and combine with other useful APIs.
Ruby, Python, Lua are in term of language design way superior. Coffeescript and co emerges because it was more reliable to have a complex stack than to use the JS language directly.
That's the only high-level language that produced this situation. Python/Ruby & co are not perfect but good/reliable enough. You don't program in a another language that compile to python to avoid its verbosity and its traps.
If python had been choosen for the browser, we would have used for the last 15 years : module, sane scoping rule, classes, iterators, etc. Yep, that's something that it ecma6 without the ugly baggage.
15 years, damn it.
So no, please give at least Python in the browser and let people experiments with other language design choices and let them find something superior to Python. At one moment, it will be obsolete like any other tool. I don't want to be forced to use it because it is the only choice.
No!
Languages (and language implementations) have different strengths and weaknesses. No one language may hope to serve all needs equally well. While for what you do your single language may be sufficient, there are people and use cases where it's better to use something different.
I'd rather embrace the diversity than try to artificially limit available choices.