Which song is "Oh Lord"? You don't mean "In the Air Tonight", do you?
But I agree on "Mama" and "Man on the Corner" -- those are actually my two favourite Phil-era Genesis songs. (Although "Second Home by the Sea" done live and "The Brazilian" is always fun too, but those are instrumentals.)
And I can't imagine Phil going up to Mike and Tony and trying to convince them to "sell out". If you listen to those albums in sequence, I think you'll see that there's a very natural progression from fantasy, prog rock to more accessible pop.
A Trick of the Tail and
Wind & Wuthering were classic Genesis albums, very much in the vein of Gabriel-era stuff. But
W&W featured a gorgeous romantic song called "Afterglow", written by Tony, not Phil. With
...and then there were three... they consciously tried to write much shorter, accessible songs -- and many of them still had a "fantasy" element. They had two hit singles from this album: "Follow You Follow Me" and "Many Too Many" -- both of which are romantic songs, the first of which was written by all three, and the second written by Tony.
ATTWT was written before Phil ever considered doing his first solo album, cos his wife hadn't left him yet! So personally, I think they were already on the road to becoming more mainstream and more pop-oriented by the time of
Duke.
But that's just my opinion.
cheers,
supervij
EDIT: Whew! I think I may have come off sounding like someone from one of those boards johndoe mentioned!