For me it’s Whatsapp. Like 99% of people i know use it so yeh… i still need it. I am considering moving the Whatsapp Gdrive backup to a local backup btw.
Google photos is also a major problem for us. We have a lot of stuff that want to migrate to different service (or store locally).
I still use Whatsapp because it’s the only messaging app that the people I know use, other that telegram and I hate telegram even more.
Luckily whatsapp is available outside of google play, it can be dowloaded from the whatsapp site, it is a bit hidden but the apk is there to download.