Every country is the world is Keynesian to some degree. Every major economist in the world today believes in the Keynesian economics. If you don't chance it you don't understand economics.The main bone of contention in stages of crisis & whether to accumulate debt.
If you read economics, you will understand without fiscal intervention & Keynesian economics, (say in Laissez faire) both the supply & demand curves move leftwards to adjust to a new equilibrium which will be at Q* instead of Q on the X-Axis.
And Q* << Q, meaning lower GDP, lower wages, loss of jobs, lower taxes which will again increase debt.
I think you're almost correct but slightly wrong. If the demand curve shifts leftwards, the supply curve will shift rightwards, not leftwards, as workers have to take pay cuts. The price level will fall dramatically, while the quantity sold will stay the same as the effects of the shifts in the two curves cancel. GDP will decrease, of course, and the rest of the stuff you said is correct.
No supply curve shifts leftwards. If supply curve shift rightwards then Q stays the same (GDP quantity similar) & price (P) falls.
Let us say say Demand has fallen 3D to 2D (example). Unemployment in 25% odd at Wages X. When wage falls to X/2 (say), less people in the labor force will be ready work the job at reduced prices. For example - When the wage of a factory is 100$ a month (say) & it is cut to 50$, the number of workers available for the job @ 50 would always be lesser than those available @ 100$.
That is why the supply curve is generally drawn as a straight line with 45 degree to the X axis. As price/wage increases, supply increases, as price/falls decreases, supply decreases.
Ofcourse this is considering the fact that workers will be willing to take a wage cut. In real life, what we see are "sticky wages" & it is difficult to cut wages by say 20% if demand falls by 20% due to union, unwillingness to take wage cuts, employment contracts, legal provisions etc.