If I want to listen to Radio Caroline in my car I use the popular Radioplayer app which pairs to my car via Android Auto. My car does not have Medium wave and neither did my previous one.
The sound quality is excellent and it's very rare that I lose signal. All the so called 'big boys' are also on that app like those from Bauer and Global. If I want to hear smaller stations I tend to use the Radio Box app which also includes what you might call Unofficial stations as well as offshoots from bigger stations like Caroline Flashback and Coastal and stations from the Netherlands and the USA.
It's not surprising that online listening has now just passed traditional listening numbers as it's just more convenient for many people with everything available from a screen or just talking to a smart speaker. No confusion from technical terms like frequency or signal coverage.
The only concern I have is that if everything is distributed by the internet , there is no alternative if there is a major outage due to a fault or even hostilities. Over the air radio will always still just work in such cases although by the time it becomes an issue, few people would know how to use and tune one in even if they had one.