Where are you getting that? It's certainly not the historical outcome.
The queen is not stepping down, she will be queen till she dies.
Charlie will only be regent till then, he gets the job he’s always wanted, she gets to put her feet up at 95 and she’s still queen.
As prince regent he does not get to ware the Kings crown and Camilla (insert young Frankenstein horse Winnie here) won’t be queen.
I guess she could die. That would change things, but she shows no signs of poor health and a complete resolve to keep Charles from the crown.