Looking at the size of alternative online communities like Parler, they're definitely not half of the voter base. Majority of republicans seem to not have a problem with mainstream platforms, only a small minority gets banned.
And it is really easy to see they are a minority. Most of alternative platforms like Parler have a small number of users and attract a very narrow audience which talks only about politics. Usually these sites have bad user experience which means there's not a lot of designers who would want to work for them, they are really bad in technical execution meaning there's not really many programmers who want to work for them. They don't advertise meaning there's not a lot of marketing experts wanting to work for them. There's not a lot of investors willing to invest in those platforms, no one wants to advertise on their sites. All this can lead to a conclusion that the users affected by deplatforming and censorship on mainstream social media is a really small group of people. They believe they are half of the voter base, while in reality the majority of society, from conservatives to liberals, rejects their world views and this is why they are marginalized.
If it was really half of the voter base they wouldn't have any issues. There would be some viable, popular alternative social media platforms used by that half of voter base. Among that half you'd have no issue with finding anything you need to have a widespead, catch-all platform - you'd find investors, you'd find companies willing to advertise, the user base would have diverse interests so that you can discuss anything on those platforms like on mainstream social media, and not just politics, conspiracy theories and memes. The fact that they constantly can't find support means that they are a fringe movement.
It's simply not possible that all investors, programmers, designers, hosting companies, payment processors, advertisers, etc. are all run by the left half of the voter base. That is simply statistically unlikely.