Infuence is not just about fans but influencing other musicians. Virtually every great band and artist after the Beatles has said the Beatles was a prime influence on them as musicians. So, if the Beatles influenced the artist, they ultimately influenced the artist music. I see endless Youtube video clips and newspaper articles where the famous artist worships at the feet of the Beatles.
A second measure is "covers". Musicians cover other bands material because they admire it and no one has had more recorded professional covered than the Beatles. One song was recorded 2000 times by other bands/musicians. You only cover music you respect.
Inventions: the Beatles influenced more inventions and "firsts" than anyone else. They invented the mega outdoor concert, heavy metal, the concept album, use of the melodic bass, they influenced culture, hairstyle, clothing. The Beatles had the first MTV style video even before MTV
They sold more records than anyone, had more number one hits, they owned the pop charts for years
They forced other bands to reach for a standard of excellence in creating full albums of good music not the one hit wonders you see today.
The case can be made that they were underrated