The trouble with this idea is that if the Monarchy ceases, so does the entire hierarchy, all of them who owe their titles and their allegiances to the Throne.
You know, Dukes, Duchesses, Earls, Lords, Ladies, etc.
And I don’t think they will let this happen, even if they have to put Meghan on the throne.
Prince Harry and Meghan Markle refused to give their son Archie a royal title. So eventually it’ll just be Prince William and Princess Catherine and their kids. That’s the Royal Family.