Open source doesn't really preclude that. They would still have control over committer rights. There are plenty of open source projects in which the majority of code and decisions are made by a handful of core contributors.
Open source does, however, require coordination time, training, answering a different set of questions than they currently do, probably rejecting all sorts of good code because it doesn't meet their vision and pissing off those contributors, or worse letting the code in.
Heck, just reviewing someone else's code is a potentially huge time sink.
