When a government, responding to the rising fiscal cost of ageing, raises taxes or tax-like pension contributions, people will adapt their behaviour, economic growth will slow down, and slower growth will generate less tax revenues. (This is a manifestation of the infamous Laffer curve.) Hence, taxes will need to be raised even more than the calculations with the global savings/investment framework would suggest.
To quantify these effects, I'd like to refer to another study conducted at the Bank of Finland. It analyses the economic implications of ageing when the economic costs resulting from tax distortions are also systematically taken into account. In that study, the economic costs of ageing for the Finnish economy were estimated in a general equilibrium macroeconomic model 2 .