If they are settling permanently overseas, with a view to being active overseas, earning a living overseas, raising a family there, entering political conversations there, I absolutely think they should not be allowed to retain the Dukedom of Sussex. What service would they be providing to Sussex or the UK? The odd conference call or Instagram post or flying visit isn’t enough to deserve the title. H is a Prince by birth, and whatever his wife and children are entitled to as a consequence is fine. But the Dukedom was gifted on marriage with a view to them setting up a household which would be of service to the UK.
If they continue to call themselves the Duke and Duchess of, while working and making roots elsewhere (and arguably using these titles to do so), they will basically be doing the half-in/half-out that HMQ has denied them.
Charles has had a place and Gstaad, Andrew in Switzerland, Margaret in Mustique etc. But those have all been second (or third or whatever) homes. They have always been based in the UK, and they could always have argued that their hearts lay primarily in the UK. I don’t know how H&M could say the same when so publicly cutting ties with the UK, fleeing, and actively choosing to raise their family in Malibu.