If you wish to be sceptical of this story (I'm fairly dubious about it myself), then fine, but Keynesians aren't arguing what you think they're arguing.

No, that's precisely what I assumed they're arguing, and I believe my points were completely responsive. I will address the position you describe in the context of the criticism in my rant.

The Keynesian theory of depressions and recessions is that excessive pessimism leads people to avoid investing or starting businesses, which lowers economic activity further, which promotes more pessimism, and so on.

The goal of stimulus is effectively to trick people into thinking the economy is better than it is, which then becomes a self-fulfilling prophesy;

Now, unpack the meaning of all of those terms, back to the fundamentals we really care about, and what is all that actually saying? Well, first of all, have you played rationalist taboo with this and tried to phrase everything without economics jargon, so as to fully break down exactly what all the above means at the layperson level? To me, economists seem to talk as if they have not done so.

I would like for you to tell me whether you have done so in the past, and write up the phrasing you get before reading further. You've already tabooed a lot, but I think you need to go further, and remove the terms: recession, depression, stimulus, excessive, pessimism, invest, and economic activity. (What's left? Terms like prefer, satisfaction, wants, market exchange, resources, working, changing actions.)

Now, here's what I get: (bracketed phrases indicate a substitution of standard economic jargon)

"People [believe that future market interactions with others will be less capable of satisfyng their wants], which leads them to [allocate resources so as to anticipate lower gains from such activity]. As people do this, the combined effect of their actions is to make this suspicion true, [increasing the relative benefit of non-market exchanges or unmeasured market exchanges].

"The government should therefore [purchase things on the market] in order to produce a [false signal of the relative merit of selling certain goods], and facilitate production of [goods people don't want at current prices or that they previously couldn't justify asking their government to provide]. This, then, becomes a self-fulfilling prophecy: once people [sell unwanted goods due to this government action], it actually becomes beneficial for others to sell goods people do want on the market, [preventing a different kind of adjustment to conditions from happening]."

Phrased in these terms, does it even make sense? Does it even claim to do something people might want?