AVRs are simple and reliable. You can read the whole datasheet and understand everything with much less effort than modern ARM MCUs. They've been around long enough that I have a lot of confidence in them. They're electrically more robust than most modern chips so they're hard to accidentally damage. There's a lot of community support available because of the popularity of Arduinos. Many tasks don't need anything more powerful. I think it would be foolish to discard AVRs as obsolete just because they're old.