I think those eras had browsers that weren't capable of delivering on the promise.

If you only work with IE9+ (all the other browsers are quite good now) you will be working in a run time environment that is far superior to those available in the DHTML/Ajax era.

Another issue is that very few people know how to do large scale JS applications, where I work we do (130,000K+ lines of code, 900+ classes) but it has taken us several iterations to get to this stage.

I believe we are planning on making our tools open-source so hopefully people will start to see that large scale JS apps are very doable. www.caplin.com if you want to keep an eye on the open sourcing of our tooling.