Well, "in-story" you wouldn't actually be buying them, but getting yourself assigned to them. Prestige is a measure of the amount of influence you have. The way I see it, the ships available in drydock are all the ships that need captains, and you are constantly being offered the opportunity to be transferred to a new command. The more prestige you have, the bigger the ships you will be offered, and the greater your ability to take your pick of assignments. Even after a ship has ceased production, it's still going to get new captains periodically, as officers retire, get promoted, get transferred to other ships, or die. Over the course of fifty years your Alpha should need at the very least three different captains, if not more like six or seven or ten.