PRs could withdraw their CPF the moment they renounce their PR. Which this effectively means that the 35% of their pay effectively goes into the CPF just enough to generate interest if 2.6% per year and they get to withdrawal the whole sum once they decided that they have earned enough and go back to their country.

this doesnt mean they "do not need to pay the price" of policies like cpf and ns. the decision to renounce permanent residency has an opportunity cost in itself which the pr would not have to make if they didnt have to make cpf contributions. and in any case singaporeans do not lose out when a pr takes his cpf money and leaves. in fact its a benefit. they would have spent their productive working years here paying the same taxes and getting less back in terms of subsidies in education, healthcare, and housing, and if they leave we dont have to take care of them in their 'unproductive' retirement years.

Also, first generation PR(s) do not need to serve NS isn’t it ?

guess what: first generation citizens dont serve ns as well