Here's an analogy. You've been saving up for a specific watch in which the manufacturer has announced he will no longer produce any more past point X. When point X is reached, there are now a finite number of those watches circulating. If you want it, you no longer (unless otherwise lucky) have the benefit of saying "your price is too high", I'll just buy one from the manufacturer.
Could you please put this in terms of investing in retirement accounts, so that it's in keeping with the thread? TIA.