I don't think copay changes anyone's career decision on a month-to-month or year-to-year basis. That decision is a longer-term process.
But I think cumulative copay over the course of a career can influence someone's decision to "Stay Navy" or AF or whatever. The catch here is that the copay has to be fairly consistent....if it's subject to wild swings every time the economy or force structure hiccups then folks won't feel comfortable planning a career based on a certain income level and I think senior leadership knows this.