In my experience, most newbies who have yet to pick a micro either want to learn programming, or electronics, or they've got an idea for a thing they want to build which will need a microcontroller; in any of those scenarios I think AVR is probably not the best answer.
So, I feel like recommending a person start with ATMEGA328P because it's a simpler architecture than ATSAMD21, is like saying that people should learn to drive in a 1960s pickup truck because it's simpler.
> Another big draw is all of the daughterboards that use the arduino headers. No other ecosystem besides perhaps the raspberry pi has anything close to the amount of compatible boards.
This isn't really an argument for AVR though, is it? All but one of the dev boards I've worked on in the last few years have either been Arduino or Feather form-factor, none have been AVRs. The one exception is a much chunkier machine.