I know this is kind of the contrarian opinion and I'm not trying to be "that guy", but if you want a web app that works in 30 years you would probably be best off building a server-side rendered application. You need a server, HTTP, HTML, and CSS for any web application, but you don't always need a lot of client side javascript.
The fewer things you have in your stack, the fewer things can change under your feet.
If you MUST use a framework, then yes i would go with ember because they have a prooved commitment to following the web standards rather than creating their own custom standards that they throw away the next year.
Having said that, 30 years are very VERY long time in web development. Maybe pure js isn't a bad call, but it depends on how large it is going to be. Someone else mentioned considering sever rendering, not a bad cinsideration either.
It has to be a web app so I was thinking of going pure JS. With that requirements in mind would you recommend ember.js?