I have been wondering about this for a while, but evil nazgul's "
what if answer" gave me inspiration.
Do youkai really need Gensokyo after all? If Clownpiece, Junko, and Hecatia are from outside (Junko is kind of weird though, i'm not even sure what she actually is), wouldn't this mean the outside world is still full of gods and youkai? Add also Mamizou and the hobgoblins, that were doing just fine without Gensokyo before Nue and Yukari brought them inside it.
Sometimes i think Gensokyo is just Yukari's plaything and that even without it youkai could survive (not sure for gods). Besides, she seems to have done nothing about Sumireko's kamikaze ideas. What if Reimu failed because she slipped on a banana peel, and Sumireko succeeded? Everything goes to hell because of that?
Another thing is Reimu being vital to Gensokyo's existence, which i believe being bullshit just like heaven being filled, or shinigami going around killing hermits and celestials (both the thing proved to be false). That's because:
-Aya's reaction in SSiB. For those who don't know, when Reimu was kept on the Moon, Aya simply talks with herself about finding someone else.
-The thing above being taken into account in "Strange creators etc etc.", where for what i could understand by google-translating it, it says it's the youkai who choose the shrine maiden (so "Hakurei" might not even be an actual lineage, but a name taken from the shrine).
-The original purpose of the barrier: sealing youkai away. If taking down the barrier was as easy as killing a single person, it means the plan was really bad made from the start.
-Rinnosuke saying that it's Yukari's power that keeps Gensokyo separated from the Outside World, and that no human or youkai can oppose her.
-Reimu going to the Moon, twice, where no one can assure her safety (add also the danger of the travel). The 1st time was even Yukari's idea, and i don't think she would risk Gensokyo to steal some sake (i know it's Yukari we're talking about, but she can't be THIS weird :V).
-PCB saying that the original barrier was made by some priests, but then youkai put "an even stronger power" at work. The power of youkai outclasses that of humans, so a youkai-made barrier could (or already did) substitute the original one.
-Reimu addressing Yukari as the "creator" of the barrier in WaHH. This is kind of weird since i'm not sure what she meant.
My theory is that they do need some poor soul who manages the barrier, but not specifically Reimu or someone related to her. AFAIK, Ran and Yukari are the ones known to protect the barrier from "erosion".
I know both the things have probably been talked about many times, but i came here only recently so i'd like to know your ideas.