I lined my Clockwork Orange codpiece thing with craft foam and it's doing okay after almost three years (I wore it the past two Halloweens).
Other people use it to make sets of armour, including people with work of quality and time investment of a high level - I'm sure if degradation was an issue they would change material.
Not trying to be argumentative, it just seems I have only ever seen evidence to the contrary that craft foam changes state significantly over time. Perhaps there are certain conditions that could cause it to do so, but it seems that costumes made from it are doing quite well years in the future.
Disclaimer : This post written by a Grumpy Old Man
Actually in the pipes:
- Remaking Ser Loras doublet.
- Luke Skywalker, A New Hope
- Wind Waker starter costume in swim material
Eventually: Paul from Dune, Faramir, Harry Sullivan