Then, when you're 25, your daddy dies aged only 56 and after a rather brief period of mourning you get pushed into taking his job in a pompous ceremony. Now you're going to be doing this until you die. No retirement! I bet there were times where Lilibet just wanted to go to her room and cry.
I wouldn't have wanted her job for all the wealth and power that came with it.
[1] Well, you could make a big scandal about marrying an American divorcee, but that didn't go down too well for the last guy.
If she had wanted to, at some point, abdicate in favour of Charles, that could have been arranged. It would have required a special Act of the UK Parliament (following the prior example of His Majesty's Declaration of Abdication Act 1936) [0] – and probably also supporting legislation in the other Commonwealth Realms (Australia, Canada, New Zealand, etc) – but no doubt the governments of the Commonwealth Realms would have obliged. It was her own decision that she did not want that. I would not be surprised if, in another 10 or 15 years, King Charles III makes a different decision, but we shall see. In recent years, monarchs abdicating due to advanced age has become rather common – the Netherlands, Belgium, Spain, Luxembourg, Japan, among others.
[0] http://www.bailii.org/uk/legis/num_act/1936/ukpga_19360003_e...
(Spain is somewhat of an exception, but Juan Carlos' abdication was linked to corruption allegations over shady business dealings in Saudi Arabia; I very much doubt Elizabeth had any such skeletons lurking in her closets.)
Her son and heir has literally been accepting suitcases full of cash from middle east despots (nominally for his charity), and her other son was up to his neck in Epstein's dealings. I wouldn't be so sure.