Implementing a limit would not solve a problem, or better said, create a new problem: the opposite.
There's either:
1)an inaccurate score that doesn't show the improvising skill of the players but shows the skill after lots of learning, practising and blood, sweat and tears. (the current situation)
2)the situation that would happen if this was implemented:
an inaccurate score that was posted 2 years ago of a mere 500k because the people who now get 1.5m were only able to achieve that high back in the days, but now their 1.2m+ scores don't count because they used all of their passes.
For example: Person A played TTFAF when AS was just released. Competition is low. Little is known about chaining, priming etc. The maximum score in those days appeared to be 500k. Person A tries to get the 1st place with 501k but uses up every 'pass' for that song he has, under the thought that his place can't be beaten because he got 5 red splats, while little was known about certain tricks. 2 years later, person B pops up, learns about chaining, priming, syncing, stuff that wasn't really clear back in the days. He masters these tricks, becomes a skilled player and tries out TTFAF. He gets 900k, almost double the amount of person A. person A however has increased his skill too and can easily get that 900k on his first run too now, but he can't get that throne back because he used up every pass, while person B still has his 2nd-30rd chance to make that 900k a 1200k.
A solution to this would be to make the passes restock after a certain amount of time, but in that case it would just be a matter of time before person A claims that spot back. Instead of having person A & B fight for that throne with 1 month in between every dethrone, why not just have them fight for that throne with 1 day in between? The 1st place shows who is better, person A or person B. While person A might be more skilled than person B, person B gets the (undeserved) spot because person A used up his passes back in his unskilled days. And in the case of the monthly restock: While person A may be better than person B, person B may get that throne for a whole month because person A got a bad day.
If I were to choose between
-scores that show a number which is way too high than people's average on a song (because of studying, hard work and learning by heart)
or
-scores that show a number which is way too low than people's average on a song (because of unskilled poor runs back in the old days)
I'd choose the one where people take effort to get their scores, not a run that could've been done better.
Making this restocking makes no sense, as a 6 month restrictions has the same effect as no restriction: People will get that throne as soon as they can.
Games with scoreboards are always played over and over again. No.1 in Defense Grid didn't get his 1st spot in an eyewink either. But having a scoreboard of ' first playthroughs' only would be senseless too because tactics can change. One can think halfway through the game:"hey I just found out about something, wish I knew that during the first levels". Or in AS case. " Hey, we just found out about something, wish we knew that 2 years ago"