Before I start - I need to make a note for readers - I'm biased towards free market and against communism/egalitarian economy based on my own education (BA in finance and banking and MSc in Comp. sci.), experience (living in communist-ruled state) and business experience (own startups, parents' business, etc.).
Both questions - regarding Trickle-down and Keynesian economics will have a significant amount of ideological bias from people who answer these. Let me answer this first ... why these questions are not "hard science" and there is right or wrong answer to any of these without additional assumptions like time (short vs. long-term), demographics, society.
Important issue was raised in the selected answer to linked question about Trickle-down economics - is economics hard or soft science. For computer scientist and math geek it's simple - it's soft. It depends from the behaviour of the mass. You could imagine going to different planet (or meeting isolated tribe on Earth) in which the answer to any of large-scale economics problems would be different/opposite than for you/your country.
This is the reason why even answering these two questions without context (country, society, etc.) is never a complete answer.
Another issue most answers to economical questions is that people support their claims with analysis of certain policies without analysis of the alternative and without analysis of the timeframe.
Above you got answer from Denis de Bernardy, who claims that QE "worked" as a support of Keynesian policy. You will find all sorts of claims like that, but without analysis the negative impact of the solution and the "what-if" analysis of the alternative, it's very often misleading.
The aim of the QE was to avoid the defaults of financial institutions which "gambled" on financial market.
If you want to achieve that at all cost, in short term ... yes, Keynesian approach works perfectly.
However, if you analyse how much money has been put into the economy and in consequence, what impact was imposed to individuals in the market in the long-term - I tend to disagree.
Keynes with his ideas towards public stimulus programs aimed at flattening of the business cycle curve to reduce the negative impact for the working class with a top-down approach. Austrian school economists on the other hand state that avoiding natural crises has negative impact as crises allow removing allocation of resources in un-efficient initiatives and parts of economy. It's the time doing "poker-like showdown" or "checks-and-balances" of investments and removing the ones that do not work.
Avoiding that creates an environment in which the inefficiencies are supported rather than removed from the economy and reallocating of resources in more efficient parts is not achieved.
The forced reallocation of resources into more efficient investments/businesses is what is causing a lot of societal problems as people lose jobs and are not able to maintain their standard of living for some period of time.
Austrian school economists support the approach in which most of the resources are maintained in hands of the people (lower taxes) which allows them to create buffer for the times of crisis (bottom-up approach). The principle behind it is individuals are more efficient in spending their own money (both for goods as well as secure-investments) than the state.
The problem with that is though, significant portion of society does not want to take responsibility in creating this safety buffer and is demanding state to do it - this is a societal problem as a lot of people tend to spend 100% of their income, regardless of how large it is.
To have a short answer to your question - yes, Keynesian approach works well in short term when it comes to relieving the potential impact of crises of individuals (less sacking when economy is down), but - in the long term it's:
- Deferring the crisis in time at the same time pumping it up, so it's a lot larger.
- Allocating resources (both money and manpower) in inefficient investments and parts of the economy in comparison to the alternative (not doing any QE-type of programs)
- Increasing long-term debt as - as stated in above answers - states are not fiscally responsible enough to save money in times when the economy is in good shape.
- Taking away the responsibility of people to act responsibly with their financial assets - this creates a negative societal results. People no longer see what is the result of being reckless with their money as state will always help.
Now going for a second to Trickle-down economics, which is in fact supply-side economics follows the principle of lower-taxes and limited regulation.
I tend to agree with Margaret Thatcher with her response to the question:
socialists' question "how you could defend policies that resulted in the gap between richest 10% and poorest 10% widening substantially. The poor were relatively less better off than the rich."
Her response was: "All levels of income are better off than they were in 1979. But what the honorable member is saying is that he would rather the poor were poorer provided the rich were less rich. That way you will never create the wealth for better social services as we have. And what a policy. Yes. He would rather have the poor poorer provided the rich were less rich. That is the Liberal (British Socialists) policy. Yes it came out. He didn’t intend it to but it did."
Everyone has to understand that in supply-side economics there is going to be larger economic inequality, but everyone would be better off in comparison to the alternative.
Supply-side economics creates and environment in which:
- There is more incentive to take risk in business, which creates more jobs, but with a little less job security as there is more fluctuations.
- Economic efficiencies are naturally promoted. Cheaper and better products and services win with the opposite, as there's limited artificial support from the government.
- There is a sense of imposed responsibility on individuals with its goods and bads.
None of these approaches is "best" and well suited for every society, but I know which one is best in the long-run. Think about it - do you know any country with Keynesian approach became rich after being poor? (Excluding ones which gained wealth mostly from mining/resources).
Last note - Trickle-down is not the opposite to Keynesian economics. They are different, but not mutually exclusive.
Apologies for a long rant (as I just discovered politics.stackexchange) and mistakes as I'm not an English native.