Keynesianism is just another arrogant philosophy based on the assumption that it's possible (and desirable) to know what's best for everybody and how to achieve it. Keynesianism has been useful because of its theory that government spending can alleviate recessions or a depression? Even if this were true (hint: it's not), how is this a good thing exactly? How do we know that a depression/recession isn't exactly the thing the people & the economy needed? Maybe because a lot of bad investment decisions were made, which now need to be liquidated (this couldn't possibly have anything to do with expansive monetary policy which...surprise, is another Keynesian recipee), or the economy needs to restructure itself due to new technological developments. Such top-down enforced "solutions" just stand in the way of real solutions.
Keynesianism is like a person getting sick and instead of lying down, ceasing activity for a while and focusing on getting better, just downs some antibiotics and goes for a run to "stimulate" his body. And then wonders why he gets cancer while being 30 years old.
By the way as has been pointed out, we desperately need economic models based on
sustainability as opposed to ones based on perpetual growth. It's sick, how talking heads everywhere freak out that "the economy is not growing fast enough" - like cancerous parasites, which just keep growing and multiplying without limit, only to eventually kill of their host and themselves. Keynesianism is just a shortcut from here to there and I spit on it