I think it depends greatly on the kind of application you're designing. A lot of web apps have a pretty simple backend, mainly persistence. If that's the case

I think it depends greatly on the kind of application you're designing. A lot of web apps have a pretty simple backend, mainly persistence. If that's the case, for some small apps a pure back end or front end dev might not have enough to do. The problem, I think, is that the front end is getting specialized, with enough churn, that it's getting difficult to maintain that specialty along with serious expertise on the back end. In the days of rails and html/css, with a bit of javascript, one dev could handle it all (and honestly, I think integrated systems like rails would be a far better choice for a lot of apps that are going full SPA). But the interesting apps I've worked on? I remember writing a manufacturing and logistics app that had a lot of linear and quadratic programming, and some fairly complicated database design and SQL, and a more elaborate front end for analysis (applets back then, believe it or not). I was pretty happy to stay away from the front end, really, and I can't see how I could have kept up with what's going on with javascript/SPA frameworks anyway.