Phil deserves this one for his legacy. He will always be viewed in Tiger's shadow because their careers overlapped but he finally did something that will be tough for anyone, even Tiger, to top.
However, let's not act like Phil didn't have his struggles in his younger days too. I don't think it's fair to write off this current group of golfers as "not being mentally tough enough" when Phil had a lot of the same question marks too. Remember, he didn't win his first major until age 34, including some memorable meltdowns. Spieth, Rory, Koepka already have 3 majors before that age and there's a big crop of up-and-comers who will be relevant for the next 10+ years and some already have 1 major too (Justin Thomas, Morikawa, Bryson).
The PGA is in a great place right now, it will be as competitive as it has ever been...which is what makes this Phil win such a big achievement. There is no one guy dominating like Tiger in the 90s and 00s but instead, there is a group of about 10-15 players who could easily win multiple majors in their careers.