That's fair if he likes something more than another. I don't think it's useful to debate subjectivity (entertaining time wasters to debate, sure, but a person likes what they like in the end regardless). I personally like them both. FTWD not as much, but I think it's still a good show, just a lot to live up to. If he likes it more, that's fine too. None of that is the problem.
The issue is blaming one enormously successful TV show for the demise of an entire genre. It would be like suggesting The Simpsons destroyed the genre of animations about cartoonists families (notice: I don't blame the Simpsons for the demise of Zombie movies). The Simpsons at peak has a much larger market share than TWD did, genre for genre(again, notice: Not saying the Simpsons is in the Zombie genre). King of the Hill, Rugrats, Family Guy, American Dad, Incredibles, Bobs Burgers, etc etc. (notice: Not suggesting King of the Hill, Rugrats, Family Guy, American Dad, Incredibles, or Bobs Burgers are Zombie shows.)