Too big to fail comes to mind, or at least "too unique of a capability".
We're seeing the old redundancy saying here - one is none, two is one.
If you're not going to be allowed to fail because then your competitor would have a monopoly, then the benefits of having "competition" is limited.
On the other hand, when it comes to extremely large projects like large passenger craft, it seems that having a lot of competition is difficult, because the global market for planes that large is just too limited to support a healthy amount of competition.