Anyway, getting to 6 majors is big even missing the US Open. He’s now in the Faldo/Trevino tier.
In a historical context, it really is astonishing how important each major win is for you:
1 - means you've (finally?) won a major, and are into the big-time as a professional golfer
2 - means the first one probably wasn't a fluke, and you're eligible for the World Golf Hall of Fame regardless of how many other tournaments you've won
3 - now you're an elite golfer: other players with three include Jimmy Demaret, Julius Boros, Billy Casper, Larry Nelson, Hale Irwin, Nick Price, Vijay Singh, Padraig Harrington and Jordan Spieth
4 - now the real stratification begins and you're approaching legendary status: others with four include both Tom Morrises, Bobby Locke, Ray Floyd, Ernie Els, Rory McIlroy and Brooks Koepka
5 - only five players in this tier: James Braid, J.H. Taylor, Byron Nelson, Peter Thomson and Seve Ballesteros
6 - only three players: Lee Trevino, Nick Faldo and now Phil Mickelson
7 - this is the first God tier: Harry Vardon, Bobby Jones, Gene Sarazen, Sam Snead and Arnold Palmer
8 - only one man: Tom Watson
9 - only two men: Ben Hogan and Gary Player
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11 - Walter Hagen
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15 - Tiger Woods
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18 - Jack Nicklaus
For Phil to graduate from 5 to 6, especially at his age and in this manner, is a massive leap in his legacy; if he could somehow get to 7, with two wins coming after joining the Senior Tour, he moves into the Top 10 (seven majors is T7 all time), but even being T12 in majors with Faldo and Trevino puts him a step above some awesome golfers. And Phil actually tied Hagen for 8th yesterday in overall PGA Tour wins with 46; he now trails only Tiger/Snead (82 apiece), Nicklaus (73), Hogan (64), Palmer (62), Nelson (52) and Casper (51). Given the era in which he played, I think you can definitely now make the case that Phil is now one of the Top 10 golfers of all time - a case that I don't think you could have made before yesterday. I'm not entirely sure I'd put Phil in my own Top 10...but it's close enough now that I'd have to think long and hard about it.