> How long should they keep data that ... the ... owner of the account agree should be removed?

Has the Internet Archive actually made a statement about that? Some Flickr support person claimed they agreed to it in a support thread, but I can imagine a lot of scenarios where there might be an unacceptable amount of spin going on (e.g. Flickr made some unreasonable ultimatum, and IA chose to delete the account instead of accept).

> If this isn't sufficient rationale to delete data, what is?

That the data is truly unwanted.