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.
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.
That's fair, and given the sheer number of games out there there's no shortage of game X you can argue is like game Y. Alex's lack of originality though has more to do with him not even being particularly creative. He's not trying to take something that was done in a creative way before and adding a new twist or combining it with other mechanics to make a good product. He's just directly taking something without understanding why it worked in the first place because it just happens to be in the latest game he was playing or show he was watching.
I'll give you that the game has generated a lot of interest, but I don't think that means it could be a successful project. The majority of fan interest is more about people's headcannons for the character and less about the gameplay. I'd compare it to the number of Overwatch fans who are surprisingly passionate about the world/characters but don't actually own the game.
I've mentioned it before and don't want to throw out a wall of text, but I think Yansim is one of those projects that is great on paper, but falls apart when you actually try and make it. Even with a competent coder, it is a lackluster stealth game that becomes very repetitive because it takes place in a single location. You can make a few map tweaks between rivals to make things harder, but there's still only so much you can do. Going back to the Overwatch comparison, this is where the game suffers, because unlike Overwatch, the game can't rely on drawing in people who are just looking for a fun multiplayer shooter to play because the gameplay is lacking. The best you can really say is it's fun to mess around for a little bit, maybe a few hours if we're being particularly generous. I think that's why a lot of the similar fan projects haven't really gotten off the ground. It sounds like those teams care more about the project than Alex, but at the end of the day, there's just not a lot you can do with the idea.
I think there's two things people like about the idea of Yansim, having to cover your tracks in a murder sim and having a social stealth game. And these are interesting ideas that could work in a game, but that game is not Yansim. I'd even argue that while both are good ideas, they don't really mesh well. A social stealth game where you ruin someone's reputation could potentially be fun, but a game like that shouldn't have any violence options whatsoever and would rely heavily on writing and creating social challenges. Stealth options could still potentially exist, breaking into someone's room to steal a cellphone or get their computer to send out fake emails or whatever, learn someone's schedule etc. A murder sim where you have to take care of the crime scene is more straight forward because that's basically just adding another layer to a game like Hitman. Maybe even do something where you just play a "cleaner" and have to react to whatever the assassin team you're working with decides to do. Either way, neither of those ideas are forefront in Yansim.
*Quick edit* I will try and give Alex some credit where it is due. I think some of his current ideas could work better under a different medium, maybe a dark humor anime or something along those lines. It would also appeal more to his fanbase which are more interested in the lore/characters and the like.