To the first point, if people only spend their earned interest they are merely rentiers, and not adding any economic value.
So is it the same in the BTC world? You have hundreds of people giving "advice" to other people here on this forum that you need to "hodl". The BTC they hold is not doing anything to the economic value of BTC other than their exchanged currency (whether that was fiat or some other means) being locked up in the system for the period they hold it. For some, the idea is that their speculation of a favorable BTC->USD exchange rate will allow them to sell or trade at some later date. But what are they selling or trading for? Fiat or a good purchased with a BTC->fiat transaction.
I fail to see the difference here, other than the POS staking in the case of XPY.
Second point, i know they can't spend the staked XPY and that is even worse. It's like locking it into a long-term CD except no bank is issuing a loan on the other side of that!
Initially correct, but loans, escrows, added services, etc. can all be introduced that create more value for the coin. BTC value creation is all tied into the POW, and speculation. I don't see any other value adds.
Third, Keynes was writing at a time when all gov't money was backed by gold. And yes, there is actually a controlling entity - GAW's reserve fund of fiat - to artificially prop up the market price. This is no different than deficit spending if you want to draw the analogy.
You'll have to forgive me... I'm not a fan of Keynesian policy. And discussing it's merits is probably best done in a separate thread in the Economics forum.