I have thought of fusion-based mining before and piezo-based energy sources coming online and what it would mean for bitcoin...

The current supply (Coingecko) of BTC is 19.297M out of a possible 21 M.

The attack vectors would be use an abundant energy source to:

1. Mine as much of the remaining BTC
2. Control hashrate to block or censor specific transactions or applications built on top of Bitcoin.
3. Control hashrate to burn / destroy things a la the novel "Dune"

If the aim is to destroy Bitcoin

1a) I see it as not a great strategy because at that point you're drawing attention to BTC and making it seem incredible valuable 1b) You're seen by the entire world as a financial terrorist in a way you can't even deny plausibly anymore

2. Competing geopolitical adversaries both engage in it, thereby proving the thesis of adversarial money, plus waves hands difficulty adjustment (someone poke holes in this). Barring an adversary, see 3)
3. Would have to destroy "last sane" snapshots everywhere all at once, no? Barring total destruction you still have possibilities of forks and network recovery