Well that's always the million-dollar question, isn't it? Take Twitter for instance. Whether there's an easy alternative to Twitter depends entirely on your reference class.
The social aspect makes this so much harder as people are unique. It's completely reasonable for someone to need to be on the same platform as some other particular person. Say, Elon Musk. Or pre-ban Trump if you want the least convenient example from a civics perspective. How do you justify kicking someone off a platform for having the wrong views if that's where their elected representatives spend most of their time?
But this perspective completely defangs your position in all but principle. Any privately-owned platform with important[1] people using it is basically any platform that has any reasonable amount of success. Even if you restrict to public officials, well, there's a lot of public officials.
[1] Note important is in the eye of the user, and people will generally self-select into platforms that have users that are important to themselves.
Just because some "famous" person uses a platform shouldn't mean the platform has to fall over itself to make sure their every word is hear and everyone can contact them.
That being said, after having a political/ideological group I really liked removed off a major platform (that it followed the ToS of) without analogous opposite-side groups being removed, I'm at least very sympathetic to your position. I thing we both agree that harmless differences of opinion shouldn't be removed, but I'll agree to disagree on whether the law should be involved.