What exactly does “should” mean in this context?
I think the implication is that it’s essentially being prevented from collapse because it’s so ingrained in international trade that if it were to collapse it would hurt you and your allies too much, so you don’t allow it to collapse when it otherwise might.
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.