Today Google announced it is deprecating it's AJAX Crawling Scheme. Google feels confident enough to rely on their bots executing your site just as if a real user were driving it in the browser. This in large part is due to the advancement of things like nodejs and phantomjs.
This decision means several things to the single page application space, the escapefragement_ becomes moot and you may not need a core site any longer. But more importantly I think it also sent a signal that single page apps were not a recommended strategy if you want good search engine rankings.
Instead I now recomend using a static web site that meets the Progressive Web App (PWA) guidelines. Proper PWAs provide that instant loading experience that meets or surpases native application experiences. The smooth navigation is the primary reason single page apps were created.
What was the Google AJAX Crawling Scheme?
If you read my High Performance Single Page Application book you know I spend a decent amount of space discussing what this specification is and how to implement a dance between the client and the server. In essence you should have a way to serve the page content as if it were 2008, from the server.
Fortunately this can also be used to support legacy browsers like Internet Explorer and old Androids. This is accomplished by doing some simple feature detection and redirecting the user to the core site. The core site is that legacy site.
So What Now?
This announcement is good news, but don't just go pulling your existing site infrastructure just yet. I mean it still works and supporting these old browsers might be important. Remember Microsoft ended support for legacy Internet Explorer versions in January 2016. This means, you should have started phasing out support for these old, outdated browsers years ago.
As for new construction you should focus on utilizing a statically rendered progressive web app. This combination should give you the best user experience that works in every browser.
Personally I have phased out my single page app investments a few years ago.
Line of business applications should not need a fall back for legacy browsers unless your corporate IT is about 6-8 years behind the times. In other words if your company or target employees have modern browsers (which you should have) you can fully leverage progressive web apps and reduce much of your development overhead required to manage single page apps.