If you need half of the abusers of Drizzle to be broken in order to make Drizzle itself broken you are foolish. Half or majority is such an arbitrary, ridiculous, and in my opinion, extremely large number that makes no sense for a number of reasons. Even if we remove Magikarp, Feebas, and Surskit from the list of Swift Swimmers, we're still left with 19 Pokemon. Add in arguably all OU Steel types, Politoed, Thundurus, Tornadus, Dragonite, Starmie, Parasect, Toxicroak, Blastoise, Tentacruel, Pelipper, and Swanna, then subtract Heatran and Excadrill, and we have grand total of 39 "rain abusers." (Keep in mind this excludes most OU viable Water types and is therefore complete BS, I just made it up for the sake of argument) If you then decide to order said Pokemon by usage you would find that Blastoise ranks 20th according to the May stats. So basically what you're saying is that we need to ban BLASTOISE, and all Pokemon that are better than Blastoise under permanent rain before we ban Drizzle. Sure, some of them in my list may not be OU material, like Swanna and Pelipper, but if we ban Dragonite (lol, 4th on my completely BS list of "rain abusers") one of them might move up to replace it or, you could instead include things like Rotom-W, Gastrodon, and Swampert.
I am much more in agreement with KurashiDragon with the idea that we set a number of Pokemon that need to be broken under Drizzle in order to ban Drizzle in general, with the number hovering between his suggested 5-8. If the number reaches and/or exceeds this point then Drizzle has clearly become the culprit and must be the broken factor. The idea that only the lower half of "rain abusers," under any context, are not broken seems pretty asinine to me.
Having said that, I still do not see Drizzle in it's current form broken at all, even with Thundurus running rampant. I'd say that it's on the same level as Excadrill, which I also do not see as broken. However, I do see both Thundurus (by extension Drizzle) and Excadrill to be somewhat "overcentralizing," in that I need to carry several Pokes that work well together to not only take care of these two threats but to also help accomplish the goals of the entire team.