On the app I get 2, maybe 3 posts per screen and at least 1 ad taking up 30% or more of the screen. I don’t see how that’s an objectively better user experience unless someone desperately needs glasses.
Another factor is that communities can’t use CSS to style their pages with the new site. /r/nfl for example is a thousand times better with their CSS than the few tools for differentiation Reddit provides.
Reddit allows you to see your subreddit's statstistics, and the last time I checked, around 1/3 of our traffic is from old reddit. So definitely not just a small minority.
(Then again, it has been around a year since I checked, so perhaps the numbers have shifted.)
The number used to be falling much faster, but seems to be falling in a somewhat stable fashion around the 15% mark over the past year.