Less Taxes: So we would have a smaller government? That's good. Government already gets so much money and waste it.
Pensions: Maybe instead of relying on a pyramid scheme, people would need to manage their investments or have kids and raise them well so they take care of them later. Sounds like a win.
Less tax revenue. Higher tax rates to try to compensate to prevent collapse.
As for pensions, arguing about what should be is not going to help the growing proportion of your population that don't have enough or that will end up having to try to help their parents and grandparents avoid destitution.
Even if people "managed their investments", with fewer and fewer working age people relative to the total population, if everyone reduced their spending to invest for their pensions, yields will drop as demand gets taken away for people to save instead of spend.
Pensions: Maybe instead of relying on a pyramid scheme, people would need to manage their investments or have kids and raise them well so they take care of them later. Sounds like a win.