^^^
Depends what the definition of “fan” is
The Beatles undoubtedly still are popular by any definition. They are currently ranked 45 in total streams on Spotify with over 10 billion streams, most of which is by listeners under 30 years of age. The Beatles have around 24 million monthly listeners on Spotify, keeping them in the top 100 on that platform.
“Waning” doesn’t seem to be quite the right word to describe their popularity when they continue to pick up listeners in the younger generations to this extent. It’s waning in the sense that they are obviously no longer the worlds number one act and that dedicated fans, ones like us who like to discuss them on forums, will probably keep decreasing. Although I prefer to think of it as a more exclusive fan base
It’s hard to know what will be the case in the future but certainly I’d agree that each generation removed from the Beatle era their popularity relative to current acts will likely decline. I’d imagine attention on them will wax and wane. I can see a time when a handful of standards remain well known by everyone but only a dedicated clique go much further into their canon. For musicians who play pop/rock I think they will remain an influence like seminal artists in all genres.
Depending on what the popular music styles are at the time might dictate what type of further resurgences they have. The most popular song on streaming services of the Beatles by a long way is Here Comes the Sun. I imagine its breezy acoustic style fits in with what a lot of the younger crowd like nowadays. If streaming had existed in the mid-90s during the height of Brit pop, the top Beatle song might have been a Revolver-era Lennon song. If pop/rock disappears then so will the Beatles, until then I can’t see them ever dying off completely.