This goes back to one of my points earlier being we are never meant to learn about the actions of all Heroes who successfully save the world, only the one that the story is focused on or want us to know about. As I said, it is all about perspective in storytelling even for those that focus on a Hero.
What Emet said and the meaning behind his words have a lot of meaning. We will never know if WoL is the only capable Hero in the world of FF14 nor will we ever know for certain due to how little we know about the world. Not to mention SE is not one to write a linear story about our character is the only capable hero in the world, just the one more well known and the one that get noticed.
Yes there will always be Heroes who failed their mission and have to delay events so the Player character arrives to resolve it for them. However, not all of them will fail and if WoL is the only successful hero with their mission then the world would have long since ended for many reasons.
This goes back to the split effect of Heroes in storytelling as well. One Hero cannot resolve all the worlds problems and not all world problems can be solved by that one Hero alone. The best the Hero can do is deal with what is in reach and aware about but if they are not aware about something then they cannot resolve it. That is where another group of Heroes become involved in which tend to end up as a Spin-off story to cover a linear event that happened at the sametime as the Main Story or a side quest in a video game for the Main Character to uncover.
One of the biggest issues a Heroes face is that they can never be in two places at once to resolve every world ending problem nor do they have all the talents to resolve everything. A lot of the world ending problem in Season 1 story focused greatly on Eorzea and nations connected to it in some way. If there was another world ending threat happening at the sametime during Season 1 in the unknown regions but due to how WoL was focusd in Season 1 regions, the world would have ended either way even if WoL stopped Meteion or not since he/she was never there stop the threat in the Unknown regions happening at the sametime as Season 1 events. You may always say the Heroes of the unknown region delay events so WoL arrives in Season 2 to resolve it for them but that is just shows FF14 world should have ended maybe a hundred times over if every world ending threat can only be resolved with WoL being alive.
Not to mention having them delayed in perfect sync to happen one after another only when the Hero of a story is alive so the Hero can on to perfectly resolve them one after another is questionable in itself as it place a big question in the story about why does all these world ending threats are set on a time in perfect sync with each other on this specific time when this Hero is alive almost as if someone or something know when The Main Character of the story (aka WoL) will end one World Ending threat so they can awakening another that was delayed by some past Hero for him/her to resolve. Of course some stories does try to cover this by revealing every world ending threat originates from some single unknown great being of power but that will be just bad writing and remove what we learned from Endwalker being there will always be world ending threats to the world but it is through people's own choices in life that leads to Heroes being born through their own hope to stop them.
Funny enough, this topic about the idea that one single being that is the source of all world ending threats and suffer (being Chaos) and people's belief that killing Chaos to end all world ending threats and suffer forever is one of the topics that Final Fantasy Origins will cover. However, that is a topic for a different FF game and maybe a bit spoilery to how FF Origin may end eventhough people can already guess what the big reveal is for FF Origins.