If anybody isn't familiar with the term groupthink, it basically means when a number of people come to a decision because each person thinks that everyone else would want to, although no one in the group actually does.
And it seems odd that by about 1977 not one member of Genesis was really playing prog. Gabriel's solo career, while undoubtedly intelligent and forward thinking, in no way conformed to any traditional expectations of progressive rock. And it need hardly be said that Genesis's efforts without Gabriel (still three members of "the" lineup) pretty much outright rejected prog - recent quotations from Collins and Hackett (is Hackett's solo work particularly in a "prog" vein? I've only listened to one early album very quietly, so I can't really judge) both contain similar sentiments, whatever the context they've been taken out of. As for Tony Banks I couldn't say, although what little I've heard seems far more to pander to a pop audience.
So my question is this: from where did the idea for Genesis to play prog come?
Can we thank "groupthink" for the happy coincidence which provided us with some years of prog music?
Did Gabriel's theatrical excesses need a prog band to match them?
Or did Hackett's/Rutherford's/Collins'/Banks' playing?
Did they all enjoy prog music at the time and later tire of it?
Or do they just pretend not to like it to sell records? 
Any thoughts?