This is an amazing deal. The Mariners are getting a star at a good price. The only other really good first baseman available in free agency this year is Pete Alonso, and he's going to get $150 million over 5 years. (I don't put Ryan O'Hearn in the same category with Naylor and Alonso although I'm sure someone will overpay for him.)
Alonso is a really good player, but I'd rather have Josh Naylor. Alonso hits more HRs, but Naylor is better in other ways. Naylor hits for a higher average and strikes out a lot less. He had 83 strikeouts, compared too 172 for Alonso this year--less than half! Alonso's WAR is only slightly higher than Naylors, 5.6 to 5.4. Also, Naylor is 2 years younger than Alonso.