Of course they can be hacked. Plausibility directly correlates with money you put towards the goal, which in turn - with money you could get out of it if successful - with risk analysis and other business/investor nonsense.
No country will officially demand a ransom because what you get is not a ransom, but a war.
Elon has said they will change network-wide encryption at once if/when they will find out that any part of it has been hacked.
Given this Elon anticipation you as a hacker not only have to hack a single sat - you have to hack entire constellation and HQ pretty quickly and first-try - before countermeasures can be applied. I do not think it is possible without first stealing extremely detailed designs of everything from SpaceX.
North Korea, China and Iran WILL ALLOW Starlink to operate in their countries at some point - as soon as Elon promises to forward all the country traffic via local ground spy centers and they can verify that Elon does keep his promise and is not to set out to "free the people" or sell all the data to US government. Likely some "test" countries with "strong" governments will be paid by China/Russia to test that. Think Venezuela, Somali, Cuba, possibly Iran itself.
