The users can choose to stick with: tyba.com/theirname or what we see with users whose name is not available they often use a variation e.g. jsmith.com john-smith.com johndsmith.com etc.
Also some users upgrade to a premium plan without a ___domain name to remove the ads and get access to the themes.
One thing: The member sites you included all have their own domains. If users can't get their name as a ___domain, what are their options?