I don't think anyone is saying that a person winning 1 major is invalid. We all agree it's one hell of an accomplishment. You can think of it in terms of anything else in life. Getting an A on a math test in HS, if you back it up with a bunch of C's, it doesn't diminish the accomplishment of that A, but if you back it up with some B+, A-, or even another A, it could be looked at differently. I know some have mentioned music, getting a #1 hit, and then never cracking Casey's top 40 again puts you in a different class than if you back it up with more appearances near the top of the charts.
You're saying, basically, do you think winning 1 major makes a player a "one-hit wonder" in golf, and if so, who do you consider 1-hit wonders?