It's interesting to consider this in the light of Drupal. Drupal has lots of contributors who are totally volunteer - and they've been contributing to Drupal for a long time. Yet, there are Drupal developers who work for places that pay them to develop sites and functionality on Drupal - even though those companies aren't the one who offer Drupal in some commercial way (the way Alfresco does for Alfresco.
I don't know whether this will change for Drupal as Acquia appears. There is no question that the community will continue to develop a huge amount of Drupal code. I'm amazed every day by how much code the community contributes. The question is: are there areas where Acquia can focus that the community won't? Are there areas where nobody that is currently working in the community is incented to do work, but which areas have some key thing to be done in Drupal?
I think there will be. For instance, Gabor is currently working on how Rich Text Editors work in Drupal. This involves some obscure work to do that has false-started a couple of times, and which we're going to sit down and "just do."
But will this type of work will be "the most" of the work in Drupal? I bet not. It may be that we may do much "important" work, but I'll be interested to revisit this in the next year to see how much code we developed (e.g. in our Carbon project compares to the quantity of contribution made by others in the same code base.
If I had to bet, I'd bet that Matt's assessment is wrong with regard to Drupal.