One party states typically exist because they are legislated into being through raw power or are imposed at the point of a gun.
Where a major political party collapses through its own corruption and unpopularity, there might be a temporary period of political vacuum in the opposition movement; but eventually, and in a Constitutional Republic there would be no impediment to the formation of new parties, a political realignment will occur.
Where a party can lose power through the ballot box, there’s an incentive to continue to act in the nation’s interest and retain the necessary level of support; so even during the realignment there is far less risk than you imply.
I think there is a strong argument for a few terms of governments of nation unity, which could further ease the transition.
But the death of the Democrat Party, and not it’s reform, is something highly desirable to many. If a center left party we’re to emerge from the wreckage, capable of forming a patriotic and Constitution-respecting opposition to the right, that would probably be beneficial to long term political life and national stability.