Please, since someone from wikipedia is probably reading this: Keep the JavaScript to a minimum. The plans aren't clear to me, but given what sites like reddit/etc have done during their "updates" the new UI's are terrible on anything that isn't a top of the line device with a screaming fast internet connection. Wikipedia (and a few other "older" sites) is such a pleasure to use as is, don't ruin it. AKA round-trippi…
The experience with js-heavy sites even on a very capable desktop is still negative. Too much js introduces bugs, latency, and lack of stability. A page like that will sometimes jump around and resize for no apparent reason. Making wikipedia noticeably js-heavy would definitely ruin the experience.
The stuff that gives JS a bad reputation is sites that slap together nested widgets that each load their resources sequentially.
If you serve a 1 MB blob of all_the_things.min.js on the first page load, then gzip reduces that to ~300 KB, which will take ~2.5 seconds to load on a 1 Mbps connection, and then it will likely be cached.
If you serve a widget that loads another widget that adds a third widget, on the other hand, elements will keep popping in and your user experience will suck.