You’re not wrong. There’s nothing that requires the two parties be Dems and GOP. But you’re not going to overturn one or the other in a single election, and that means losing to the farthest big party from you, likely a few in a row, while that gets resolved. Especially if you try to do it top down instead of building support from local/county offices up.
Basically, if you could get enough third party support, you could either supplant one of the existing parties or force them to shift to stay competitive. The argument is that trying to do so with the office of president when doing so promotes a fast track to outright fascism is a painfully bad tactic.
The way I always explain it to people - take any additional government power or access to information you either don’t care about or actively support. Now imagine whoever you oppose/hate the most taking office and trying to use that against your interests. Are you still OK with them having that power? Same principle applies regardless of what power or who’s pushing for it.
It’s like due process - you don’t want any category of alleged violation not to be subject to due process, and if you don’t understand why then it’s time to wrongfully accuse you of doing that so you understand the problem.