Having a possibility of failure isn't a lockout. There's no requirement of certain characters or anything. You'd be able to complete more maps as you gradually level more characters and give them equipment. The increasing rewards from events should speed up team growth. Weekly quests for more manyu/ampies and a second tier of affection for more raw stats should be in the near future, too.
There'd also be more manyu from ultimate maps and more seeds from higher level story raid bosses if Nutaku ever adds them.