A computer is a physical object. A hard drive contains data, physically, much like papers in a safe contain data, physically.
And it can be forcefully opened, too - it's merely a matter of magnitude of effort. Cracking a safe could take hours or days, cracking a password could take millennia. It should not be my problem that the government has locksmiths with drills on staff, but not supercomputers capable of breaking encryption.
I wouldn't expect the case law to be predicated on the fact that most safes can in fact be opened even if the combination is not supplied, specifically because, as you point out, there are safes that would destroy the evidence in such a case. The justification "meh, who cares if he gives the combination, we'll just open it forcefully and obtain the evidence anyway" doesn't hold for, probably the more important, cases.
An example of this is a retail drop safe -- it can only be opened by specific people at a specific time.
At the end of the day, you need to weigh the options. If you are an attorney performing your ethical duty, you have an obligation to rot in jail while the illegal warrant is litigated.
I disagree. A safe is a physical object that contains other physical objects. It can be forcefully open.
OK, maybe there are some very sophisticated safes that would act exactly like a password, not allowing to extract the objects inside without damaging them beyond recognition.
But I'd say it's the exception. Possibly helped by the safe manufacturers, the police will eventually access the contents of most safes.
This situation creates a different set of incentives. You can open your safe as soon as the judge requests it, or you can wait to be punished for refusing and the safe be opened forecefully later... that will uncover what you were hiding anyway.
Edit: I'd like to add that the punishment for not telling the password is a very, very, very bad idea. It's impossible to produce a password that you don't know. It's impossible to demonstrate that you don't know a password. It's impossible to demonstrate that a file is just a pile of random garbage instead of an encrypted one. Setting a punishment for things that are impossible to objectively know doesn't seem a good idea.