Moved from the Sussex forum.
Yeah, I think it's the timing that matters here. I'd love for some Brits to chime in on this: could they reasonably have intended to remain as full-time royals while living in a Commonwealth country, perhaps with the intent of preserving the Commonwealth in the future? If they'd been so inclined, I think they could have done a lot to keep Australia and New Zealand "in the fold," so to speak, while being based there. Maybe if Australians and New Zealanders had their own royals who did things like hand out youth awards and open senior centers as a full-time job, instead of just popping in for a tour every few years, they'd attach more value to the monarchy and be more in favor of keeping it around. At least, I could understand the palace and Harry/Meghan thinking along those lines.
Maybe it would better for some Australians, Canadians, or New Zealanders to chime in as their opinion in this case would matter more than that of the Brits. In any case, if I may give an outsider’s opinion, I am not sure if having royals living full-time in a Commonwealth realm would actually help or hurt the monarchy in those countries.
The Queen already has her constitutional representatives in the realms, namely the Governor General and, in the case of federal realms like Australia and Canada, also the state governors or provincial lieutenant governors. Those officers also perform the ceremonial duties of the Crown in their respective jusrisdictions, such as giving out awards for example.
Up to the mid-20th century (?), Governors General e.g. of Canada or Australia were often British peers and sometimes even royal princes (like the Duke of Connaught or the Duke of Gloucester). The nomination of native Australians or Canadians to the vice-regal office, especially when Australian or Canadian citizenship became legally separate from British subject status, was an important landmark in the affirmation of the realms as independent, sovereign nations and I think it is something that the realm governments value immensely.
Members of the Royal Family other than the Queen are not given any explicit role in the constitutions of Australia, Canada or New Zealand. They have patronages in the Commonwealth and perform ceremonial duties on behalf of the Queen during offiicial royal tours/visits, but that is not considered a full-time occupation. If a royal couple, be it the Sussexes or someone else, moved full-time to a realm and were given a permanent state role, that role could possibly clash with the duties already performed by the Queen's vice-regal representatives and, even worse, could be seen as a "neocolonial" setback. I certainly imagine some republican groups would try to paint it that way, whether it is actually fair or not.