Metro operates bus lines to other counties, so I don't think there would be a jurisdictional issue (or issues could be ironed out) there if they hypothetically wanted to run the Gold Line all the way to ONT. Whether that's financially viable is another matter.

I always thought the Metrolink station was too underutilized and has so much potential for ONT, but the Riverside line is the redheaded stepchild of the route network. Perhaps building a new station at Grove & Mission + a timed airport shuttle service from the Upland Station would help?