if they'd choose to live quietly in the US i don't see a reason for them not to perform some royal duties, but imo they seem to be on a path to not persé live quietly, but actually get involved, get a platform and have a public opinion.
This choice imo will have the logical followup that they can't do royal duties.
This is not a problem for them, it is their choice.
I could imagine HM and P.Charles regretting it a bit, but they too won't have a big problem with it, the RF will adjust and the others will divide the jobs amongst the people who have remained loyal to the royal service.
If a monarchy can overcome a king stepping away from his duties, it will definitely overcome one of the other senior members stepping down.
It is unfortunate that this coincides with another royal (P.Andrew) having to drop royal duties, but things in life don't always happen when and how you want, so this too will be managed by the RF.
It is not H&M's fault that the Andrew thing happened and if H&M don't see themselves fit or willing to live a life of service, they should step out rather than continue and be a liability within the RF.
Imo this is probably why there was a 'trial year' to see which direction H&M's choice will take them, and imo their appearances lately and their speeches and involvements show they choose a path bringing them further away (not just geographically) from the RF.