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