The idea of a number as big as 70 was to not really inhibit that sort of tactic unless you were doing it for minutes on end (and if you were, it was ledgestalling with a different name all along). The only real effect this rule would have would be to weaken real ledgestalling; basically any other use of ledgegrabs has no hope of reaching 70. It's reasonable to argue that ledgestalling doesn't need weakening (I'm not convinced it does), but that's pretty clearly about the only thing this rule affects. At worst, you could say that Yoshi is allowed to ledgestall less than other characters because he likes to ledge cancel his eggs as an attacking option. That isn't beyond the scope of the current rules really since our arbitrary percentage tiebreak system is already biased against heavy characters a bit. If I'm at 81% as Bowser and my opponent is at 80% as Jigglypuff, I was "winning" in the sense that my opponent is closer to death than I am, but the tiebreak system makes me lose. It's not a big deal, but it is a consequence of the rule to be aware of.
Anyway, the "system" JonaDiaper is proposing is almost funny since he's basically advocating the archtypical fuzzy rule. Yes, rules are subject to interpretation and uneven enforcement; that's inevitable. However, we don't strive to promote that in law or in competitive gaming. Like, we can't have IRS agents auditing every person's taxes every year. It's not reasonable. However, at the same time, we don't make our tax law "pay about 10% of what you earn, but pay less if you have tough circumstances and more if you're rich" with people being held in line by judges. Since it's possible to concretely define taxation rates, the tax code is very specific about how much you owe given any possible circumstances. An IRS auditor can't be "nice" or "mean" since there is extremely little ambiguity in the tax code. If someone finds some abuseable loophole and ends up paying radically fewer taxes than they should, that's fine. Their actions were within the law, and they will not be punished for it except perhaps by a change in the law so they can't do it again next year.
It is true though that some laws are naturally bound to be more fuzzy than others. Take harassment. If I call you 20 times a day for a week and cause you great distress, that's probably harassment. However, what if you're an extremely close business associate from whom I really need information 20 times a day for a week? What if we have extremely poor cell phone service and keep having to call each other back for dropped calls? On the other hand, I could only call you once a week but say deeply disturbing things when I do, and that would be pretty clearly harassment if I kept it up after you told me to stop calling you. This law is definitely fuzzy, but how could you even define harassment explicitly if you wanted to? It's all about the effect it has on the person you are making contact with, and it inevitably must be up to interpretation.
Now take the case of ledge stalling. Is it more like tax code or harassment? The question is whether you have some way of quantifying how much ledgestalling the opponent is doing. What a coincidence; the game has an internal counter for ledge grabs. It's entirely possible to rigidly define ledgestalling. Yeah, we might have some loopholes where people can ledgestall just some and get a lot out of it, but overall we can make it a pretty clear rule that will adequately cover the "problem" that those who want to make rules against ledgestalling see. Why would we opt for a fuzzy rule? What advantage does the fuzzy rule have? It increases the need for judges both numerically (they have to see the game in action, not just the results screen) and in terms of qualification (my little sister could make a ruling with a hard count rule, but she wouldn't be able to make good subjective rulings). It increases the variation in terms of judges and adds a level of partial treatment (you even admit this!). I see no gain and a bunch of losses; why would anyone ever support that?