The franchise is almost 90 years old so we retire one every decade, on average. Strahan's 92 should be retired as should Carson's 53.
If people worry about running out of numbers, the best answer is to use non-official numbers (such as three digits, 156; or use alphanumeric numbers, like A6) when you are on practice squad or haven't made the team yet, and when you make the 53 man final roster you get a traditional 2 digit number. Problem solved for about 300 years or so.
As for unretiring a number, BLASPHEMY. It's there for people who left an indelible mark on the franchise, for all time.