Frustrating your consumers by not being able to supply their orders is how you stop being a monopoly. Eventually your backlog is long enough that it becomes worth your clients figuring out how to do what you do in-house, or a startup spots an opportunity. Being oversubscribed is a nice position to be in, but it doesn't mean you can rest on your laurels. (Plus, even if you're hitting your profit targets, leaving a boatload of money on the table has gotta be frustrating for at least some of ASML's employees and insiders).