I always ran endgame with the same ~12-18 people in FFXI, after the initial years of HNM stupidity. Of course, you could do super large linkshells if you wished but there was little point to it, especially during the pinnacle Abyssea years. Nothing in Dynamis besides Lord required more than 18 competent people, either.

So I guess I'm not sure what old-school MMOs we are talking about, but running the same content with the same people is how I've rolled for the last 10 years.