More revenue is always relative to growth, and growth has long been in a decline. It's why we moved to a fiat system to stimulate with artificial growth.
Many markets that drive the west today boomed once being opened to the free market, but most of them got their start from public spending. Market's are boom and bust, drive the cost down and commodities something then all of a sudden you're no longer growing. If other nations are increasing public spending on research then we'll have to compete there equally.
The reason for taxes is to raise unmarked funds that can be allocated to something new, existing revenue is already marked, and it sure as hell isn't going to be unmarked if the revenue is coming in from private spending. Unlike taxes which have to be raised and voted in, the quantitative easing that bolsters the market is borrowed directly against the people outside of any tax framework.