I'm 73, and yes, Grand Funk sure were a big band for awhile and they had some good songs, but I'm not sure about the 'standing the test of time' thing. I saw them once around '71 or '72 and this was back in the days of loud, distorted PA's, and my ears were ringing for 3 days afterwards.
And at 73, I am not at all hard on current music; I'm more apt to be hard on stuff that was popular when I was growing up; today I listen to contemporary rock and pop music, among others, but I don't think I've listened to Grand Funk in over 45 years.