"Majors" is a much-too-easy device for evaluating greatness in golf, not only because which tournaments are "majors" has changed, but because of depth of fields. Golf is sort of unique in that way, and it muddies the water at least somewhat. Bypassing the issue of the US Am, just the question of whether it was easier or harder to win the PGA at match play rather than stroke play makes the comparisons difficult.
So maybe looking at two less easily defined "ideas" would be better? You might call one "peak value", and the other something like "sustained excellence". So Johnny Miller might be the poster child for "peak value", while you could take Sam Snead as the prototype for "sustained excellence". You can substitute any names you wish here; I don't care. It's the Mantle-Mays comparison; nobody was ever better than Mantle was at his very peak, but nobody was ever better than Mays was over a long career.
Looked at that way, instead of just the device of "majors", some things change at least a bit. There is still a very small group that clearly defines the top of the mountain; Nicklaus, Woods, Hogan, and then others that can be argued either way. But the arguments quickly become referendums missing legs of the majors, and I just don't like that. I just don't see Palmer or Snead or Mickelson as somehow diminished, and if McIlroy or Speith each win one more tournament in their careers that happens to the be the right ones, that they were somehow immeasurably greater. Or less if they don't...
In any case, Mickelson is in the second group after the Nicklaus-Woods-Hogan guys. He's now been truly excellent for over 30 years, which is Snead like, and his "peak value" is among the highest in golf history as well. And he did this not only against very deep fields, but during the exact era when Woods was winning golf tournaments, both majors and non-majors, at a rate unlike anything in golf history before or since.
I don't know if that makes Mickelson top ten, or top 15, or whatever, and fwiw I'm not especially a fan of his anyway. But there is NO measure of greatness that he's missed out on in terms what what he's accomplished, and quibbling about the US Open doesn't change that. I've always objected to evaluating athletes by what they have NOT done, and golf is no exception.