Oh cool so my magical internet money that I’m meant to use as my main currency is essentially untrustworthy as an actual store of value, because I’m supposed to keep migrating to whatever the hasn’t-had-a-51%-attack-yet flavour of the month currency is?

You missed the point.

What if the attacker is invested in other cryptocurrencies? Think outside the box.

Governments are the only entities who could try to attack Bitcoin if they collaborated. The only motivation would be to stop the network, not to make gains in other cryptocurrencies.