Unfortunately, RedHat tends to be the trendsetter in the Linux world. Once it's gone from RHEL, it'll be gone from most other RPM based distros soon enough. It's kind of a shame that one company has grown to dominate the Linux software ecosystem, often to the detriment of all involved.
On the contrary, RHEL ships a relatively small subset of packages, and is especially stingy on kernel features in order to make the distribution supportable.
Btrfs is definitely not gone from Fedora, for example.
(Disclaimer: I'm on the virtualization team at Red Hat).
I don't think that number is even close to accurate. SLE almost certainly makes up more than 15% of the market alone. And that's ignoring all of the other enterprise distributions.
I believe that 2015 estimates from the IDC[1] had RHEL at ~60%, SLE at ~20%, Oracle Linux at ~12% and "Other" at ~8%. But I can't access the document at the moment.
Number of installations is not also necessarily a good metric. Red Hat is a very large company with a lot of money and and a lot of employees working on upstream Linux projects and driving their direction.
There are not that many companies doing the same. Which is why they have a lot of influence over the direction of things.
