I think most people don't need full feature parity. That's why a lot of these techs take off. They need something that is easier to understand.
Similar with kubernetes, there's like 5 permutations of software deployment people actually want to do. Deployments are one of those but there are still a few areas that you have to get hacky with StatefulSets to achieve. Once some PaaS-wrapper for kubernetes takes off we'll see the V2 of that idea as a stand alone cluster management thing. If someone nips some ideas from MaaS [0] we might even see on-prem bare metal take off again once people realize they're getting screwed on perf/bandwidth and many of the benefits of cloud providers are already provided to you by something like Kube.
[0] - https://maas.io/
Git is very simple. There are basically just four concepts: blobs, trees, commits, and pointers to commits.
However, git is far from easy! The UX is haphazard and confusing and many people are scared of venturing outside of their couple of memorized commands.
As far as I remember Mercurial was easier to start with but Github came along and won.
HTML5 is definitely not simpler than Flash was. I'd argue, it's far more complex, if you include all the sub-technologies and frameworks required to reach feature parity.
There was also no "natural" progression from Flash to HTML5. Oracle and the browser vendors had to run an involved depreciation campaign to get developers off flash and move them to HTML5. If you had left the web ecosystem on it's own, my guess is, we'd still be writing flash applications today.
> Perforce, svn, mercurial -> git
This progression was definitely natural, but I doubt again that git is less complex than svn or mercurial. However, one big advantage of git is that it is extremely easy to set up: You can just install it and use it locally without bothering any admins or setting up any servers. Once you are already using it, it's also easy to connect your repo to a server.
So, I'd argue, git has better UX and better availability than the alternatives, but it's not necessarily simpler.