I use it as an alternative store of value. That has gone badly for me in the past month. But its supply economics make me still want it.

No, because energy is not free.

Energy expended to mine BTC in the past is a sunk cost and so is irrelevant to the discussion. In the future, if the price of energy required to mine a BTC is below the value of the BTC there will be an incentive to mine BTC. There is no equivalent incentive in the opposite direction. You can convert energy to BTC, you can't convert BTC to energy.

What happens if the value collapses bellow the point where it's profitable to mine it then? Wouldn't this only reinforce the collapse as miners flee the system? Then once it goes below a threshold where it would be trivial to attack it and take it over it's a free for all for the remaining market value and that would be the coup de grace...

Real question, has anyone actually developed an attack that could be executed? I know “what it would look like” is easy to explain, but is there any implementation?

I’m not sure we’re super susceptible to an attack. Assuming you could spot an attack on the network post-hoc (anyone can read the blockchain), if an attack occurred it would destroy the value of bitcoin. This would probably destroy the value of the attackers coins and the attackers mining rigs… which seems like a net loss.

If no one mines, the difficulty decreases to where you can mine on a laptop again. People will do it then, like they did at the start. A 51% attack is designed to be less profitable than just mining it. But maybe some gov does it to try to shut it down, well then, the chain gets forked and we go on with our lives. That's the point of everyone having their own node.

So what? The value of making a transaction just needs to be high enough.

even if there is an energy cost, for some markets (illegal substances/darknet), it's "cheaper" to pay that energy cost than the cost of laundering fiat money.

Indeed, providing something money-like without central authority (thus circumventing regulation, so far) is the only innovation of BTC.

I mean, that's neither true nor what I said but good attempt.