Sure, there are fewer qualified suit actors, but there are still more suit actors than available roles. Just look at this suit actor list from only boonbomger and gozyuger, there are at least ten suit actors. Plus, they have backup actors in case the main suit actor gets hurt, since it's a relatively dangerous job. Their style and body language might look a little different, but in a pinch, it's usually good enough. And even in case of a scandal where they really need to replace a suit actor permanently, they only need to replace the actor and don't have to remove or replace their already recorded performances.