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 fourth is catnip. Always the 'nip.
Once my mom asked me to clear out a planter box that had become overgrown because she wanted to plant tomatoes. I pulled up this 3ft tall stalk plant with little purplish flowers, and as soon as I shoved it in the trash bag, the cat came running from the other side of the house and dove face first in the bag. After a minute or so, it was backing out of the bag with the stalk between his teeth dragging it out of the bag and once it was out of the bag started nibbling on it and rolling around on the plant. Looked up what the plant was and apparently I had just pulled up his catnip grow operation. When he came down, he was mad at me for a few days.