That there's some intrinsic purity to the older material that only fans who have reached a certain level of enlightenment such as yourself can appreciate. You have grown wise enough and sophisticated in your tastes to recognize that even if you enjoy a song like Paperlate (to choose a representative example), it's a 'blind spot', and the song is somehow objectively garbage despite your enjoyment of it. This argument is drivel of the lowest order. There is no inherent value to one song over another, only different attributes that you might enjoy more than another person. I personally prefer Firth of Fifth to Paperlate but is that because it's a 'better' song and I know what a good song sounds like because I'm a smart person with refined tastes? No, because that would be bullshit. I just prefer it because I like a long dramatic song for one, and I love the melody in the opening piano bit and the atmosphere in the guitar solo. Could I understand someone preferring Paperlate? Of course. To each their own.
I'm not entirely sure this is what Lazlor was saying, though I'm sure he/she will confirm or otherwise. But regardless, I really like how you express this as it certainly does nicely rebut a certain kind of old prog-head attitude I see and hear often. As you said, it's one thing to say "Wind & Wuthering was the last proper album they did" when that means "I don't like anything they did after Hackett left" which, although it's not a view I share, I think is fair enough. But when it comes with this much more dogmatic assertion that everything post-Hackett or post-Duke is definitively rubbish because it isn't Cinema Show or Vine and it earned them hit singles and huge audiences, that's what I have a problem with.
It's on the same part of the spectrum as another gripe I've heard before, that when they packed out stadiums and mega-arenas on the IT tour it mainly wasn't "proper" Genesis fans, and only "true" fans would appreciate the old songs in the set. Now that really is arrogant codswallop.