Retirements are already included in the above calculation of the current rate of upgrades.
Retirement of a captain may create a captain position for the next FO, as does an increase in the total number of aircraft, but vacancies do not create captains. It is an upgraded FO that creates a captain.
At the current rate of 49 upgrades per year, with no change in manning levels or training rates and assuming that 1600 SOs and FOs ahead of that last pilot on the list, it will take 32.6 years for the last pilot on the list to complete his upgrade.
Besides the minor variables affecting this estimate a little bit, what's wrong with this calculation of the current time to command?