Denis stated in 2007 that he is in F1 to win...not to play happy families. Yet, despite this, he got rid of the best driver in F1. In footballing terms, this would be the equivalent of Guardiola getting rid of Lionel Messi, because he doesn't get on with him.
If you sack/mis-manage the best driver in F1, as a manager, you are no longer capable of doing your job.
Remember, the best drivers have huge egos and can at times, behave like children. As a manager, you have to understand this and must have the ability to get the best out of your drivers. Ron Denis no longer possessed this ability and within months of getting rid of Alonso, he resigned as head honcho.
Compare this with Whitmarsh, who arrived and within months, he had signed up the reigning World Champion, to partner Hamilton (something that Denis would probably never had done, as he wouldnt want Hamilton to ever come under pressure from his team-mate (which is why Denis had Heikki in the No.2 car, even though we all knew he was terrible).
My suspicion is that if Denis had quit sooner, Alonso would've stayed at McLaren for at least another season or 2.
If Denis came back, I think Button would probably be gone and the No2 driver would be some chump who Hamilton could beat without problem - basically Heikki, mk2.