Even if we assume that Keynesian economics is accurate and correct (it's not!), and that the economy is driven by demand rather than supply (it's not!), how does any of that negate or disprove tax cuts?
If giving people more money allows them to better stimulate the economy, then why not give them more more by letting them keep the money they would have spent in taxes? The poor and middle class will have more money to spend on goods and services, while the rich would have more money to invest in said goods and services.