flash/sliverlight then html5 then wasm.
soap then rest then json api.
Flash/Silverlight/HTML5/WASM - this, to me, is a story of a technology being useful, then growing to give too much power to web publishers - who tend to abuse any given power - then reverting to a weaker technology in order to unscrew the web. Rinse, repeat - HTML5 is already rapidly approaching peak Flash, and introduction of WASM isn't going to help here.
Soap/REST/JSON API - you could call it a story of simplification, but I don't get how it even came to be. That is, how XML gained so much popularity, given that simpler and better tools for almost all of its uses were already available and known.
1. You could represent it as a nested structure of your language's standard lists & maps, which you already know the API of, which you could directly operate on. For dynamic languages, this is much faster to stick a prototype together, even if it bites you in the ass eventually. But by the stage, you've already chosen JSON.
2. It was less characters to manually type.
3. Particular uses of XML were extremely verbose. The S in SOAP stood for Simple, which looks ironic in retrospect.
4. In a time when payloads didn't routinely use compression, the closing tags could be a noticeable increase in size.
5. The vast majority of communication that now uses JSON didn't benefit significantly from XPath (people prefer to navigate data structures using their language features, not a generic API), namespaces, DTDs, XML Schema etc.
In about that order.
You could argue that much of this is superficial, and it is, but the industry has shown time and again that lowering the barrier to entry, even in ways that make little difference in the long run, usually wins out.
6. Too easy to accidentally reinvent Lisp via xml.
I thought it was funny the first time I ran into an instance of it happening, which quickly turned into horror. Not only was the logic split, but it meant mentally parsing this:
<If><RegEx string="{AZ*}"><Register /></RegEx></If>
I don't quite get this part.
Both HTML & XML cater for DOM models. I think this is & was appropriate.
Browsers had support for executing XSL stylesheets because it made sense for a lot of use cases.
You are free to choose from a great number of useful and fun pre-approved apps that you love. The possibilities are limitless!
(Of course, you must choose the same apps as everyone else. You are a loyal citizen, aren't you?)
Which is exactly why we the cynics say we've come full circle.
Also, while the cyclical nature of client/server design seems to be a thing, unfortunately underlying ownership is not cyclical. For instance, a cycle or two ago, the "edge" computer meant software I bought running on my machine. This cycle, it means software I don't have any stake in running on machine I lease and have little control about.
This is the part of the trend that's worrying to me. Cycles of thin/thick clients are irrelevant. Disenfranchising end users is a problem.