I don't disagree but as somebody who had been a fan for several years, the change of direction was quite a shock. I always attribute this to the increased influence of Phil, whose writing seemed to be going in a different direction more towards the shorter popier songs. The band clearly enjoyed the extra sales and exposure this change of direction gave them. I agree it's quite possible without this they would have split. I heard Abacab but didn't like it and so Duke was the last studio album I bought. I have continued to enjoy them live and continue to buy their live output.
Definitely not Collins's "fault":
Banks in November 1980:
"The idea on the next album is definitely to be slightly more ambitious than on the last two, just in purely musical terms. We'd like to try and have some rather longer pieces, and perhaps some shorter stuff as well. Certainly things that are a little more experimental."
Rutherford in the same interview:
"We've discussed the possibility of doing a double. Just an idea. I'd love to do it."
And Rutherford again, after the album was released:
"Most of the stuff that didn't go on [the double album] was stuff that was most like Genesis. Most [were] tracks I could say, "That's a bit like an old song," and that's a worrisome thing for a band...
"Had Abacab died a death commercially I wouldn't have blinked an eyelid, because Genesis at the moment is so strong. Stronger than we've been for a long time."