I’m aware of few people who use SMS in Europe, and very few people who use it as their primary means of texting; I’ve even seen people outright ask that they not be sent SMSs. WhatsApp is almost ubiquitous, and it often feels like it’s assumed everyone has it, even if they don’t use it as their primary texting method.

It does seem very common in the US, however.

They tie themselves very closely with Google services, to the point that they refuse to be on FDroid by design

While they do push the Play Store version, they also have an APK, and my understanding is that it does not rely on Google Play Services, though it might be buggier without them. If I recall correctly, the origin of Signal not being on F-Droid was related to the building and signature model that F-Droid used (builds by F-Droid, then signed by F-Droid’s keys), as mentioned in, eg, this issue. With that said, it has been pointed out that there are alternatives, like a separate repository, than, eg, the Guardian Project uses, and F-Droid apparently does now have a process for developer-signed apks.

Their general hostility toward outside developers and forks, however, and that the awkward server side code availability seemed to be related to the brazenly problematic cryptocoin advertisement, are extremely disappointing, however.