Werdum certainly deserves to be in the conversation for HW GOAT. Victories over Gustafsson, Overeem, Cain, Hunt, Nogueira, Big Country, Fedor, Big Foot, Brandon Vera, Gonzaga, and Aleksander Emelianenko should at least get you in that conversation. Particularly because he submitted more than half of those guys.
Having said that, at 43 years old im not sure how much more he can offer as a fighter right now. He has perhaps a 2 or 3 more solid fights, I guess but he is obviously well past his prime already. Aging legends are sometimes a bit sad to see, and aside from a Fedor rematch (which not a lot of people are very interested in seeing happen again given their current situations) there isnt much more for him to do in the UFC or Bellator. Signing with the PFL was definitely the right move for him, though. He has a very good chance at getting that million bucks within a year. Aside from Isaev and Goltsov, he will not have a lot of difficulties in the PFL.