This is so interesting to read!
I live in SE Asia and have a full time, live in nanny, but that is normal here and I am definitely not loaded or a celebrity!! It's interesting comparing your way of working (and high salary for crazy hours and commitment) with what we have here.
There are definitely families here who seem to go through nannies quickly, sometimes through bad luck and sometimes because they have mad expectations of the ladies who work for them and who are tight with their pay and conditions. Some treat their nannies (and other helpers) appallingly. 24/6 would not be normal here, as people who want that would have a day and a night nanny, but my nanny worked for a family before me where she didn't even have a bedroom of her own, she slept in the children's room so was effectively 24/7. This isn't legal here but nobody cares about domestic workers' rights.
My nanny works 12/6 (ish) but with significant breaks and flexibility, though she rarely asks for any flex at all. Now they're all at school her hours during term time are probably more like 9 or 10 hours a day, as I don't ask her to help me get them up and out in the morning or her hours would just be too long (though lots of people told me of course she should do this...)
I love her dearly and while I pay her probably mid-market rate, I overpay her in bonuses, always invite her on trips (and she can say no, but she loves to travel), and I genuinely treat her as a member of my family. I will never, ever understand trusting someone with your babies/children and not respecting them and treating them well!
Most nannies here are uneducated, many are migrant workers. I had my nanny do a first aid course but apart from that, she has learned as she goes and she is really fantastic. She can wrangle 3 kids on public transport like a pro! She's picked up tips and ideas from other nannies in our neighbourhood and just learned from watching how I do things, as there was a bit of a language barrier in the beginning so communication was tricky. Obviously all this is very different from OP and other highly trained nannies in her world, and yet they do similar work. Nannies here are paid a very low wage from a western perspective and often send most of it back to their own families. Many have children of their own, being raised by their aunties or grandparents while their mothers work. It's heartbreaking but culturally totally normal here.
Someone mentioned about parents perhaps being concerned about their children bonding more with the nanny than themselves. I know some people here who have this concern but it has never crossed my mind. I'm a single parent and I'm a big believer in it taking a village to raise a child. The more loving, supportive adults a child has in their life, the better! I actively WANT my kids to love their nanny (and they really do). Especially as we are far from any other family, grandparents etc.
As I said, this is all quite different from the OP's world, but it's very interesting for me to see the similarities and differences in how it all works. Now I've had a nanny, I can't imagine not having one, and I will be eternally grateful for this young woman who has seen me through some serious lows with total loyalty and discretion and kindness, and has given my children so much love and time.