To be fair, making a successful or 'good', whatever that means, game isn't necessarily about being original. Just because an idea's been done before doesn't mean it's been done in a specific way, or combined with other ideas in specific ways, or in a specific context, etc. In the right hands I think Yandere Simulator could actually be a very successful project. It's generated a ton of interest even with progress as slow as it's been.You could argue he didn't even really have an original idea in the first place. Everything is based off of someone else's ideas/works, originally Hitman but swallowing up a bunch of other games/animes/other weeb shit. The one concept you might be able to genuinely attribute to Alex is also one of the biggest roadblocks to the project, the panty shot currency thing. And even then, the only reason it manages to be original is because no other game dev was dumb enough to try and include that as a mechanic for obvious reasons. Hell, there might even be an argument that basic concept was lifted from Yakuza 4/5 where you got new moves by photographing stuff, it was just made pervier by Alex.
Now, Alex is not the person to make Yandere Simulator. Whatever he's contributed, he's painfully limited by his own lack of talent as a programmer and a developer, as well as terrible people skills, and his ego appears to be too large to allow him to see his own limitations.