While the impressive number of over 300 packages quoted on the Ubuntu bug seems like a very hard task, distributed across many contributers, I can't imagine this being a blocking issue. As for 3rd party apps: sure, could cause a third party app to break. But the same app will be broken on any other issue.
Even if the Debian folks absolutely do not want to risk breaking something, how about at least using the shared memory aspect of the cache? You can stat every directory in /usr/share/icons to make sure the cache is valid. But if it is, you'll save 300kb per process by using the cache.
Anyways, it looks like the spec is staying as is. The folks upstream have given (IMHO) a well reasoned argument that the spec is implementable and reasonable.