You do realize that Wartime spending that occurred is an example of big time government spending right? It fits exactly into Keynesian models.
Most conservative economists may claim FDR's new deal programs "didn't work", except for the fact they did: The US economy was recovering when WWII started. It's just a historical fact. You may have faith that conservative trickle down economics works, but you can site zero examples of it ever working, and other than revisionism that ignores the historical record of what occurred with the New Deal, there isn't anything to stand on. I also love the fact you have to ignore with a wave of the hand the fact that WWII wartime spending is exactly the big time spending Keynesian economics calls for during a depression: If Conservative economics were true then WWII spending should have broken the country...
It's not rocket science. The extremely wealthy do not spend as much of their income on consumable products as the middle, profession, or lower classes. Lowering the tax burden on the extreme upper class in effect shifts the burden to the actual consumers, and that reduces consumption. Less consumption, less demand for production. Keynesian economics is simple and it works.
The real problem with US economic policy is the fact we have shifted the tax burden onto the professional class. We pretend the top 10% of earners are somehow the super wealthy and pretend the super wealthy don't have an income and let them basically ignore taxes. Professionals with 6 figure incomes are overtaxed, I'll agree with you there. 7 figure and up incomes are extremely under taxed, it should be like the 60s and 50s, where the extremely wealthy are taxed at near 90%, that would raise income for the government without harming demand in the slightest; all that would happen is rich people with bitch. We had incredible economic growth in 50s, and 60s with an extremely high tax rate on the top income earners (really it was close to 90%) and that did correspond to the fastest economic growth in the nation's history.