I still prefer cork to screwcaps, for the sake of tradition and aural/olfactory pleasuring.

It is only a matter of what you need. You can put on 10 inches thick of stainless steel, but your craft won't ever make it. Plus, the steel might have a flaw (besides weight) in it that the cork does not. There are different steels, different metals, different plastics, and probably different corks to choose from. If standard cork is used, it is probably the #1 material for the job. If not, NASA should be overtaken by the Mighty Cheese People.

Cork is an absolutely acceptable insulating material. It just sounds like they have a QC issue.

Listen, in the early days of the space program oak planks were used as a heat ablative material in that their ignition temperature is so high that by the time one burns off you have expended a great deal of energy.

Don't be so quick to discard the use of common materials.