Billions of Computers Run Linux and Many Use Debian (or a Derivative of It)
THE official Debian site says: "Debian will run on almost all personal computers, including most older models." Former Debian Project Leader Bruce Perens worries that systemd puts at risk the "universal" part. He is probably not wrong.
Debian runs perhaps billions of boxes if one counts miniature devices or embedded (here's a 'census' from 6 years ago, taking notes of STBs; it cites this page that says about 2-3 billion devices ship each year). If one counts Debian derivatives (such as Ubuntu) as well, then that's even more. But keeping an accurate count is hard and many devices never get updated or even communicate with the Net, so exhaustive tallies are infeasible.
Debian makes it to the top 6 in Distrowatch (not just this past month, also previous months), but does not make it to top 6 in Steam Survey unless one counts Ubuntu as 'Debian':
Debian is very widely used and many other distros depend on Debian. If possible, Debian can be "fixed" (social issues) rather than replaced. Some "soft" forks include Devuan GNU+Linux, but this past week it just about made the top 50 in Distrowatch. There are distros that are based on Devuan, including a version of Peppermint OS (currently ranked #38).
Without any potent 'crystal ball' it's hard to predict what will happen in the next 5 years. Who ever imagined that SUSE would pick up a fight with OpenSUSE, seeking to effectively squash it like IBM did CentOS? █