Nice Pictures.
The American small radial market, like the Pobjoy was fading fast in the very late 30s.
There were some European small radials including a 40hp cylinder one around 1930.
But
most of the aircraft owners in the 1920s-30s-40s were a pretty practical bunch of people and the Flat-4 and Flat-6 engines came to dominate the market and even pushed out the inline 4 and 6 cylinder engines. Using 5 cylinders when 4 would do or using 7 cylinders when 6 would do was not going to last very long.
The Popjoy used small cylinders running fast to get power and required a reduction gear for the propeller.
They may have been smooth and nice to fly behind but the cost to overhaul them would have been higher than the larger, slower turning radials and the flat fours offered lees vibration than the inline 4 cylinder engines.
A lot of the engines in the 1920s and 30s were going through major changes ever few years. A/S was building these in the 1920s
View attachment 663443
Note the much cruder cooling fins compared to the Tiger engine of the 1930s.
Also note that the 1920s Bristol Jupiter not only had the crude finning of it's time but used a different cylinder head from the later Mercury and Pegasus engines. The Jupiter used parallel valves and not the inclined valves of the later engines for an easy identification feature.
What people choose to fly behind for nostalgia maybe a lot different than what they would choose to fly behind in a "working" aircraft like an air taxi or doing flight instruction and racking up hundreds of hours a year.