[zaphar]

    after the initial boost of productivity, the effect will 
    no longer be noticeable

But the purpose of the team isn't just to get the initial boost but also to prevent a continual loss of productivity as the tooling inevitably bitrots. When you hit a certain size that bitrot happens much faster than you might think. It makes sense to keep an EE team on just to do maintenance. And support future changes to workflow. New languages and frameworks. Large scale refactorings happen more and more frequently as you hit walls that weren't there a month ago.