If you invest wisely, you will make money or approximately tread water even if the economy is shrinking, because businesses operating with sufficient foresight make money on average. It might not be enough to retire on. That is a separate question. If there is not enough juice in the system to fund everyone's retirement, then neither investment nor taxation can create that productivity. There might be some in-between solutions but anything that does not involve generating enough production and profit to at least fund pensions seems guaranteed to be regrettable.

Re: Taxation, if you want to make up for the shortfall in people's pensions with an additional tax, you're assuming that someone else is making enough money to make up the difference. That is not the case. People can hardly fund their own pensions to the extent recommended, much less anybody else's. Most businesses are not profitable enough to fund extra pension contributions either, and imposing that cost will stifle them.