Am I the online who uses Firefox and old.reddit.com to browse reddit on my phone? It works pretty well.
I am not sure if it is statistically significant but I founded a small tech sub-reddit (~30k users) and from the moderator stats 70% of traffic is from their iOS app.
I used to use 'reddit.com/.compact' which was an actual mobile friendly and light interface, but that was depricated this year and now, like you, I am stuck with old.reddit.com on mobile.
But for 'direct' access basically if I put new.reddit and old.reddit and 'mobile web' whatever that is, the total of all three categories is less than 10%.
I guess also that these stats will not be very accurate for desktop if people are using ublock, ghostery, etc.