A lot of them are actually putting the wealth into foundations that their children will end up controlling. I believe Warren Buffet has done that.
So even though their children might not have all that wealth outright, they still have the influence and wealth that comes from control of that wealth through the foundations.
And if they do, if you believe in meritocracy, it won't matter as that wealth will be squandered away with each subsequent generational transfer, as is often the case.
The best reason for a meritocracy isn't fairness or justice, it's the benefits it creates for everyone. When someone smart is given control over resources, they create more resources, for themselves and others.
I trust the creator of wealth (small, medium, or great) to allocate it intelligently far more than I trust a bureaucracy to do so.
They've taken advantage of the schools, infrastructure, courts, laws, etc.
It's a social contract, it's two ways, and part of the price of all that stuff they've been using is to pay inheritance tax.
If they don't want to pay, they could go to a country without all those benefits. But they shouldn't expect to be able to take everything with them, it's not just theirs.
As for not working as hard, plenty of rich people have recently declared the majority of their wealth won't go to their children. So certainly won't have a 'devastating' effect as you claim!