There is nothing forcing the allowance of campaign season debate on these platforms, I think it's reasonable to put our society above the health of these platforms and allow formal debate to only happen on the mainstream platforms - people can talk on other streams, but campaigns funding (either directly or through friends) discussion on those platforms can be pursued.

Bear in mind, it's not like we have no control over debate platforms now - the rule enforcement has been lacking but foreign money in campaigns is strictly verbotten, free speech has some limits.