You take a person that could be producing goods and services in the private sector, and pay him to work in the public sector, where he produces no such goods or services, and often is tasked with creating and enforcing regulations that limit economic freedom.
Clearly you're going to end up with lower economic growth.
The fact he would spend his salary on purchases is small comfort, especially when he'd likely earn a larger salary to spend in the private sector.
It's a false premise that taxation does not produce a good or service. E.g. USPS, defense contracts, farm subsidies, etc.
It's not clear that you're going to end up with lower economic growth. For example, taxing some level of income that would otherwise not be spent on economic activity (e.g. savings or equity investments) and spending it on things that do result in economic active results in higher economic growth.
Clearly you're going to end up with lower economic growth.
The fact he would spend his salary on purchases is small comfort, especially when he'd likely earn a larger salary to spend in the private sector.