Currently the HRHs are:
Philip, Charles, Camilla, William, Kate, Harry, Andrew, Beatrice, Eugenie, Edward, Spophie, Anne, Richard, Brigtte, Edward, Katherine, Michael, Marie-Christine, Alexandra and a soon to be Baby Cambridge - a total of 20.
In 20 years there will probably be: Charles (84), Camilla (85), William (51), Kate (51), 2 children (20 and say 18), Harry (48), a spouse c40), 2 children (c10 and 8), Andrew (73), Beatrice (44), Eugenie (42), Edward (69), Sophie (68), Anne (82), Richard (87), Brigtte (86), Michael (91), Marie-Christine (88) - a total of 21 - so an increase not a decrease if Harry's kids are HRHs.
Add another 20 years and we will probably have something like: William (71), Kate (71), 2 kids (40 and 38) and 4 grandkids (16, 14, 12, 10), (if the eldest is a girl it will also pass on the HRH as the heiress apparent while a younger girl won't - so I am suggesting 4 grandkids with HRH - a girl first and then a boy) Harry (68), spouse (60), 2 kids (30 and 28), Andrew (93), Beatrice (65), Eugenie (62), Edward (89) and Sophie (87) - a total of 17.
That is hardly a reduction over 40 years with Wiliam only having 2 kids and each of them having 2 along with Harry having 2.
I am assuming of course that The Queen's children have similar longevity to Philip and herself and that Andrew doesn't remarry a much younger woman and have more children - who would also be HRHs (and I still think that is a possibility for Andrew - I can see him marrying a women 20 or so years his junior and having two more children, particularly ttrying for a son to inherit York)
I do think some people don't realise how young the Gloucester are as one suggestion has them gone in 20 years - they are only a year or 3 older than Camilla and Charles as are the Michaels of Kent. These cousins of The Queen may be the same generation but they are nearly 20 years younger having been born in the war years - gap is similar to that between Peter and Zara to Louise and James who are closer in age to Peter's children despite being the same generation as Peter.
Note that I haven't included Louise and James into the above lists because they don't use the HRH but if they are added then there are 2 more in each count.
I have also obviously come up with fictional ages for Harry's spouse and children but that is simply to put some sort of idea in there - he may never marry, or marry a woman closer in age or older or....
I have also only assumed 2 children for each of William and Harry but they could decide on more which would add numbers or fewer in which case the numbers might be lower. I have also given only 2 children to William's 2 children but again that could change between 0 and whatever.