Supply side economics works if everyone is poor and there isn't enough stuff for everyone, the assumption is that the supply side can never be saturated and there is always excess demand (think of a hedonic treadmill). It works fine in developing countries with a young population.

As soon as you have a developed nation with an aging population where everyone is scrambling to save for their own retirement, you get the exact opposite problem. People are deferring spending, which means deferring incomes, which means deferring jobs which means unemployment.

There is lots of money available to invest into businesses, in fact, people are investing too much, interest rates fall through the floor. Low interest rates allow unproductive companies to stay alive and when the long term debt cycle ends they all die at once. That's not good for your retirement.

When you expect to retire in 10 years, you want your savings to actually be able to buy things, by making sure there are people in the future willing to work for your money. That's why you invest your savings, to make sure companies exist that sell stuff to you in the future, but how are those companies supposed to survive the 10 years until you retire, if you never buy anything?