But what will occur as those older, more experienced employees leave or retire?
Nothing will happen; as it has little to do with older and more experienced employees. Superbly reliable automation in today's airliners has led to a far greater safety record than the "old" days and while today some regionals employ CPL cadets as first officers on turbo-props (rightly or wrongly) , the experience level of new first officers joining the major domestic airlines is relatively high.
Go back 45 years ago and GA pilots were getting into Ansett and TAA DC3's with less than 300 total hours and not even an instrument rating -crewing aircraft that were piston engined and nowhere near as mechanically reliable as present day jets.