Just like Lady Gaga discovered 2010s pop, and Britney Spears discovered 2000s pop. Billie Eilish is the pioneer of 2020s pop music. The mellow soundcloud sound is the sound of the decade. This started blowing up in 2019. 2019 was very transitional for music and started 2020s pop music.

She had potential to be the next superstar back in 2019 but it turned out to be a nothing burger. She faded out of popularity after 2020 has nowhere near as much fame and influence as Gaga or Britney. Most songs now are retro pop or pop punk rather than whisper pop.

Au contraire!  :) "Happier Than Ever" is one of her biggest hits. I'd say she's as popular as she's ever been. I'd say Halsey is less popular than she used to be (asterisk). All of her albums have been Top Two hits, Hopeless Fountain Kingdom being her #1 record. That said, her song with Juice Wrld "Life's a Mess" peaked at #9. Manic shied away from the pop direction of Kingdom, while If I Can't Have Love, I Want Power is a full blown return to her Badlands sound.

Even more off the radar would be:

a) Lorde - Last hit (2017) "Green Light" #19; last charting single (2020) "Solar Power" #64. She was always more alternative pop anyway.
b) Meghan Trainor - Last hit (2016) "Me Too" #13; last charting single "Nice to Meet Ya" (with Nicki Minaj) #89

Billie still books the big gigs and the big talk shows.

Man, no wonder modern music sucks.