Hi Pete,
The jury is still out on whether the capacitor would have eventually burst.
I think it's fair to say that electrical/electronic components operated beyond their specified operating conditions are more likely to fail within the MTTF that's been established by the component manufacturer. The problem we face when replacing any component, that's not a like-for-like substitution, is whether the component we fit actually meets or exceeds the specification of the original one.
Assuming the designer has selected a component to give an acceptable overall MTBF for the system then if we select a component that's not up to the job then at best we reduce the reliability of the system and at worst it may fail in a way that's unexpected, e.g. pop, bang, burn, etc.
Reliabilty testing in service is not a particularly good way of checking if the choice of a component is Ok in my opinion (particularly not higher power components) - you didn't say whether the incubator had been put back into service or not - capacitiors can give off some nasty fumes and smells when they vent off that're likely to be toxic if they start to burn.
At least the designer has the luxury of testing during R&D to make sure their choices do not adversely affect reliability or cause harm and it's their job, I suppose, when all's said and done. Personally I'd use the justification for a lack of a replacement component to force a purchase since, invariably, I've found that when push comes to shove the NHS can always find the money to replace equipment if it's really needed.
Just giving my views and opinions, that's all.