I managed to skip the whole Angular/Mongo hype, and now that I'm actually getting back into frontend development, it's all about React. Can't say I missed it.
I remember telling my boss at my very first job "Yeah, I want to learn MFC and COM". His response was "Why? By the time you're out of college, everything will be .NET." It turned out he had underestimated: by the time I graduated from college, MFC & COM had been replaced by .NET, which had been replaced by webapps running under J2EE, which were just about to be replaced by Rails & Django.
.NET and J2EE (or C# and Java) never really replaced each other--at best, .NET was MS's attempt to upgrade the MFC/COM stuff to the Java VM world after J++ fell apart. They're more or less the same model, one's just the Windows Server flavor while the other is the Linux/Unix flavor.
I'd say peak J2EE was 2002-2003 - it continued to be a factor up through 2007 or so, and probably still is now but the hype has long since passed it by. .NET had a peak hype cycle around 2000, when it was announced, and also continues to have many satisfied users today but ceased to gain major mindshare right around when Google started ascending in 2004-2005.
React is a nice piece of software, I'm glad to see it introduce FRP to a mainstream audience, and it fixes certain problems around componentization that you will run into if you try to build a big-enough app in vanilla JS. I've been tempted to use it for a couple startup ideas, but then I remember that no app is "big enough" until it has actual users, and getting them is the hard part. So I remain largely agnostic about its usefulness for startups, while believing that it can be quite useful for more established teams.