Multiple Mapped Domain - Primary?

If I enable to allow multiple domain mappings per site, why is there no option to choose which is primary?

Let's say hypothetically, I mapped:
google.com
google1.com
google2.com
google3.com to my domain and I wanted all of them to redirect to 1 domain, like google2.com, if I wanted.

In other domain mapping plugins, like the main one non-wpmudev people use, they have this option, I was hoping to see that added in v4.

Any ideas without doing DNS trickery? My users often will change their primary domain, so it makes sense to allow this option.