You lose if you sell or if Trump fucks up the economy enough to completely wipe out many businesses (such that their stocks go to zero/get delisted).
Frankly, given the real possibility of (a) the US becoming a Russia/Iran/North Korea-style pariah state or (b) the MAGA fascists triggering a civil war, that could actually happen.
Remember, This. Is. Not. Normal. The usual economic assumptions do not take into account this sort of sovereign risk, and thus may not actually apply.