Seeing as many facilities around the country have gone the route of retiring their geriatric elephants to a sanctuary or a retirement facility in recent years, I wouldn't be surprised if Milwaukee end up doing the same.
Inherently it's just easier in a lot of cases to send to a sanctuary - AZA expectations and activists being what they are, if you end up down to one or two elderly elephants you're sometimes in the hot seat with tough options. The AZA obviously doesn't hard and fast enforce the three elephant rule (Bronx, and til recently Phoenix) but it's definitely preferable when possible. Given Milwaukee hasn't really been the center of much activist criticism (far as I'm aware anyway) I'd imagine they have some time to weigh the direction forwards.