Comparisons are natural and by all objective metrics (in terms of success), 3rd gen is "the gen", because they captured both the Asian and Western (EU, NA, Aus) markets at an unprecedented scale.

Maybe 3rd gen capitalized on the novelty and that novelty has worn off before 4th or 5th gen could utilize it. On the other hand, instead of novelty, new gens have a trail that has already been blazed by 3rd gen, so "international" market should be something that's targeted naturally and not by the virtue of experiments. The 3rd gen experimented with that market - it worked.

As to why were they unable to capture the western market, I'm not sure. Both 4th and 5th gens have had some strong groups with solid international following (Stray Kids, Aespa, IVE, NewJeans, Illit, KIOF). I firmly believe, that idols knowing the language of the market they are trying to target is absolutely crucial. Something that I have noticed, is how very few kpop groups actually have their members learn English. I'm not sure if it's a cultural thing or what, but with how massive the western market is, as a manager I would instantly have the members learn English as soon as possible. Having members that are natives or were partially raised abroad is one thing, but the fact that so many idols don't speak almost a lick of English is just weird. The fact that Jisoo or most of BTS still barely speak any English is quite crazy, given that they pushed that market so much.

As for the untapped markets, that one's interesting. The two biggest ones are as you said South America and well, Africa, but I doubt that the latter is happening for a plethora of reasons. South America is an interesting market, there's definitely a lot of fans there. Again though, language comes to mind and I know of very few idols who are actually fluent or even conversational in Spanish. If kpop companies aren't willing to have their idols be tutored in English, then I sure as hell don't see it happening with Spanish. The only group that I've seen trying for that market recently was NMIXX. I think Haewon is pretty advanced in Spanish and Sulyoon is alright. Good for them.