For now. And then because not enough people install them they will be bundled by default. This has happened over and over, for instance with sync, pocket and so on.
The whole idea behind test pilot stuff is to trial stuff intended for future inclusion in the main distribution, getting it force-fed to be able to claim a large installed base is the logical next step.
You say that, but I've heard nothing about Tab Center (the Test Pilot vertical-tab sidebar) being folded into newer versions of Firefox. Luckily, a community member forked the old code and ported it to be a WebExtension so it still works with Firefox Nightly, but it really needs to be built-in to the browser for maximum polish.
They are trying to move away from the old extension API (because maintaining that ate up a crap ton of resources, so don't come up with that move being a waste of resources) and Tree-Style Tabs is not yet compatible with this new API.
The Vertical Tabs test pilot thing was them demonstrating that it could be done (including to themselves, they need to test these APIs) and giving the community something to work with, should no other tree tab extensions crop up before the deprecation of the old API.
>maintaining [the old extension API] ate up a crap ton of resources, so don't come up with that move being a waste of resources
Arguably not a waste if that meant preserving the most powerful extension environment out there, and also one of the very few reasons for people to stay with Firefox.
"Not compatible" is just another way of saying "we don't have the resources and/or the inclination to support your programs". Look at what Microsoft does to ensure backward compatibility. They simulate specific windows bugs for specific old games that were coded to expect the original bug in Win95. Just so users' programs keep working. It's a matter of priorities and resources. And I believe both are out of whack at Mozilla right now.
It was only not long after that when Mozilla's fresh direction decided to start pushing the WebExtensions bandwagon only while dropping everything else, in spite of the thousands of man-hours invested by the addon developers (many of which, you can understand, were not happy; see http://fasezero.com/lastnotice.html )
>they had to ditch the old extension model
To me, that's more like a political decision they made at an arbitrary point in time.
EXACTLY. This is just like that little IM Client 'ello' or whatever that came bundled in the last version. I used it for a few days and said exactly what you did as I uninstalled and went back to a different browser.
The good thing is that these are all separate browser extensions, none of which are bundled by default. So that's exactly the case.