Why would that be unthinkable?

If bitcoin does threaten USD status as reserve currency, then it makes total sense to attack the network, precisely because it has no nation backing it, an attack wouldn't seen as an attack on x nation's security.

It doesn't even have to be the US and its allies, it can be China and theirs if BRICS currency or RMB do become something significant. In fact the latter is extremely likely, since one of the main selling point for bitcoin is free capital movement.

All you need to see is if the interest for them align (yes it does), the potential conflict (low), cost (high but not really in grand scheme of things) and effectiveness (which is pretty damn effective if lightning cannot function well)

An example would be carbon neutral and ESG policies.

Not to mention the multiple attack vectors can occur at the same time, crack down/capturing miners, stronger restrictions, spamming network etc.

Just like funding ukraine is profiting off a proxy war against Russia, attacking bitcoin is protecting their prime export asset, USD.

As any bitcoiner should know, trade off isn't measured in any monetary value as profit/loss, it's power and always have been.

If anything, a network that is unusable for a sustainable amount of time, will be far worse than 51% attack.