I always thought that if I found myself marrying a Prince then got given a Duchess title I'd be pretty pissed off.
Little girls want to grow up to princesses not Duchesses.
Duchesses are old, and there are lots of them randomly about, and definitely imagined to be below princesses in most people's imagination. Certainly in America.
I also thought Sussex is a crap place to be a Duchess of. It's non descript and doesn't have weight.
TBF I also thought Cambridge was a crap one too.
The good ones are York and Edinburgh.
Prince and Princess of Wales are the most dashing glamorous titles though.
Especially for Kate being the next to hold the title after Diana. It has such resonance.
So let them be duke and duchess of Sussex if they want to. I don't think it's going to change much for them except make them look like their cosplaying being royal in America.