[tshaddox]

Is there any evidence, even anecdotally, that reducing datacenter costs had any significant relevance to the development and rise in popularity of single-page apps and JavaScript UI libraries like React and Angular?

My own memory of the history of the web is that early web sites with a lot of client-side interactivity benefited greatly from the huge leap in interactivity, and came out several years before JavaScript libraries were widely available for ordinary developers to build web sites with similar functionality. (I'm not counting arguably "non-web" technologies like Flash and Java applets.) The earliest such web sites I can remember are e-mail, calendar, and mapping apps.