After all HRH was supposed to be for life as well.
HRH is for life. Harry is still HRH and will be until he dies, unless the rules are changed by Charles or William which I can't see either of them doing. Harry is a male-line grandchild of the monarch. He will be the child of the monarch at some time in the future.
George VI made it clear that even Edward VIII, after the abdication was still HRH The Prince Edward as he would always be 'the son of a monarch'.
Harry has agreed to not use HRH. He hasn't lost that style and won't lose it.
Diana and Sarah lost their HRH. Diana was the mother of a future monarch.
Harry gave up use of the HRH because he was told if he didn't it would be taken away.
Harry was not told he would have HRH taken away. He will always be HRH as he was born HRH. He was only told he couldn't use it for commercial reasons - same as Beatrice and Eugenie.
The only way he could lose HRH would be to change the rules to limit HRH to only the heir apparent in each generation ... so Harry, Charlotte, Louis, Andrew, Beatrice, Eugenie, Edward, Anne, Richard, Edward, Michael and Alexandra would also have to lose it. HRH won't be removed for one person.
Diana and Sarah gained HRH by marriage. The Queen took it away when they divorced.
When the Dukes of Kent and Gloucester die, you said their sons' dukedoms will cease to be royal. How far down their male lines will the dukedoms be retained? Could these titles go on for generations as long as there are male heirs? These usual royal dukedoms could be out of the running to be granted again to a new generation?
Both titles have male heirs for the next two generations - The Duke of Gloucester has a son (Earl of Ulster) and grandson who will in turn be Dukes of Gloucester. Assuming that Lord Culloden (the Duke's grandson) has a son then the title will continue.
Kent has more male heirs - his own son and grandson - Earl of St Andrews and Baron Downpatrick along with Lord Nicholas Windsor and his three sons and finally Prince Michael and Lord Frederick.
They last as long as there are male heirs. Kent was only royal since Queen Victoria's father in any case. Gloucester is ancient, and will cease to be royal, but there are loads of other traditionally royal dukedoms to be granted: Kendal, Clarence, Ross, Albany, Edinburgh.
And I would like to see a Duke of Cardiff ,Caernarfon,or Belfast. Cumberland and Windsor should probably be avoided though, since they have poor associations.
Albany, like Cumberland, is unavailable. Although removed by the Titles Deprivation Act 1917 from the then holders there is provision for any future descendants to claim them back. Both have multiple male heirs.
Carnarfon is already a Marquisate - subsidiary title for the Duke of Chandos so not going to be turned into a Dukedom. It is also separately already an Earldom.
There is also a Baron Cardiff of Cardiff Castle as a subsidiary title to the Marquis of Bute.
Edinburgh will, in the normal course of events, go to Edward - when both The Queen and Philip die the intention, announced on his wedding day, is that Charles will recreate the title for Edward.