Combining hash functions is actually trickier than it looks, and some people are doing research in this area and deploying solutions. See https://crypto.stackexchange.com/a/328 and https://tahoe-lafs.org/trac/tahoe-lafs/wiki/OneHundredYearCryptography. It does seem that if cryptography people had more of a security mindset (that are not being defeated) then there would be more research and deployment of this already.
This post is a container for my short-form writing. See this post for meta-level discussion about shortform as an upcoming site feature.