I wonder though, if she can just say "I'm working on my UK citizenship, I plan to become a dual citizen, blah blah blah" and then not work on it at all and let time pass? Though, the Queen could call that bluff and tell her they could fast track her citizenship so that she could retain her Duchess title. Perhaps that is not possible though for political reasons? Maybe they can't fast track it and she has to go through the usual process which includes a general residency requirement of 5 continuous years (thanks, internet)!
When they first got married, that wasn't likely considered an issue. They thought she'd, you know, actually live in the UK since she married Harry. So, the 5 years would pass and she'd get her citizenship. Now, its a huge stumbling block when she will likely only spend a few days each year in the UK. So, yeah, the Queen likely could move on removing her title at some point, but I think she is letting events unfold.