That's still four or five times the key not being depleted, and the lower level key not being available in the group finder. It may seem like nothing to you, but it's actually a enormous number. If every group decides to try at least 3 times before depleting, we still have 3x less lower level keys in the finder we'd normally have. Then you deplete, successfully complete the lower one - and what now, try the higher key again several times before depleting? It's 100% certain that with that system, we'd have an excess of higher level keys in the finder, and in turn lack of lower level keys.
In a healthy system, there's more or less an even number of all key levels being run simultaneously, so no matter what skill/gear level you are, there's a healthy amount of runs going on for everybody to find a spot. That's exactly the case at the moment:
https://www.wowhead.com/news/mythic-...eason-1-348645
As you can see, there's a wide range of keys to choose from; and keys of all ranges are being run all the time. This is also due to the deplete system; changing the system to only deplete on will would upset the balance in favour of high level keys.
In the current system, keys gained are random, so running your own keys is not that effective for score. If you're unlucky, you will simply not get the key you need, so doing your own key is not really a good solution here. You will often want to join other people's keys; and you want those keys to be available in spades. DPS already have to spend considerable time looking for m+ groups if they don't have a great score. In a world where less low level keys are being run, it would only get worse for them, as they would be competing for less spots to work on the score.