There'd probably have to be a very compelling reason either to switch video player again, or to allow multiple built in options (given anyone may theme in what they want). That'd be something like mediaelement no longer being supported, focusing on commercial users, lacking some major feature, getting a bad reputation for some sin, etc.
I'll leave this open in case such a situation arises.

Fair enough. I imagined at some point that Composr will be moving away from flash fallbacks and shims when support for older browsers is dropped, which is why I said v12 (or somewhere down the line). If MediaElement is still viable when this happens then great ;)

I'm not following, but it may be a hole in my knowledge. What is the relationship between Mediaelement and flash fallbacks and shims?

MediaElement has Flash fallbacks, as shown in the screenshot for the addon and the footer of their website. Shims are mentioned a lot in their usage.md though I am not sure if they are used in the Composr addon/v11. I recall you opted for MediaElement over VideoJS for this reason (backwards compatibility) at the time the addon was created.

https://compo.sr/data_custom/images/addon_screenshots/mediaelement.png

From https://github.com/mediaelement/mediaelement

"MediaElement.js is a set of custom Flash plugins that mimic the HTML5 MediaElement API for browsers that don't support HTML5 or don't support the media codecs you're using. Instead of using Flash as a fallback, Flash is used to make the browser seem HTML5 compliant and enable codecs like H.264 (via Flash) on all browsers. In general, MediaElement.js supports IE11+, MS Edge, Chrome, Firefox, Safari, iOS 8+ and Android 4.0+."

I see, fair enough. I've tagged for v12 as indeed I can't see why we'd bake in a media player designed primarily to mitigate on a browser's lack of HTML5 video support. We're long past that.