GB wrote

There is no way I can do what I do now when I am 70 at the same speed.

Could I still do it? Why not?

It won't be a matter of whether you want to work at 70.
It will be a matter of necessity (for most people).

I don't buy that.

However you slice the cake, you have retired people and
working people, and all pensions (whether funded or not)
involve a transfer of income from workers to retired.

That doesn't mean that most people will be working at 70.

If that imposte becomes too great, the workers
will simply revolt and refuse to pay it.

The tax system doesn't work like that.

Apart from an annual cull, the only way to keep down the proportion
of retired people in the population is to raise the retirement age.

That's not right either. The other obvious way is to
accept immigrants that have more kids than the indigenous
population does. And that's precisely what the EU does in fact.

Even the Japanese, who have much lower levels of immigration
than anyone else, don't get the effect you claim is inevitable,
even when they have had a depression for 20 years now.