I suppose it depends on what it takes for the police to be allowed to forcefully open a safe. If the police obtains a warrant to search your premises, are they already allowed to forcefully open safes? I don't expect so, since they have only established 'probable cause' and are looking for evidence, contraband or what-have-you in general, but are not looking for anything specific of which they can reasonably suspect it is hidden in the safe.
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.
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.