Sure, 156 more Pokemon contributed to that (such as in Gyarados' case) but the addition of weather has negatively impacted usage, more so than any changed metagame in the past.
It's not Gen 4 anymore, the past viability of a pokemon has nothing to do with banning/tiering efforts in Gen 5. I wouldn't be opposed to banning a weather, yes I'm still against Drizzle, if it was broken but I think we're gonna need more than, "the best of Gen 4 OU aren't as good in Gen 5."
Pokemon are judged solely on how they compete with weather sweepers. If it loses to more than one regularly, it sucks. End of story. No one uses it.
Weather sweepers are the current threat in the metagame. Weather is about 40% of the metagame this Gen. If a pokemon loses to 40% of a metagame regularly then yes, it sucks in that metagame. No one should use it if it loses that often.
If viability of previously-unviable Pokemon is an argument to NOT ban weather, shouldn't it work the other way around as well?
It's a fact that weather has in some capacity been contributing to diversity in this meta but that's not a good argument not to ban weather IMO. If weather makes previously bad pokemon good, and good pokemon great, then weather might be overpowered. You touched on it in your post and I'll elaborate my thoughts later.
Weather has made some of last gen's top threats (ignore Aboma; different example) borderline dangerous to use without weather around it. Heatran is useless in rain. Its nearly useless in sand.
What about Sun? Softens a crippling water weakness and boosts it's already powerful STABS. Besides, a pokemon's viability in Gen 4 has absolutely nothing to do with it's viability in Gen 5.
'Chomp wasn't even broken in Gen. 4 (imo).
Lmao!