By your logic "it's not secure, it will eventually be cracked" then private/public keys are in the same boat, no?
Yes but not the same threat. Cracking ancient spent private key keys harms no one, thus no problem with keeping transactions on the block chain. Cracking ancient anonymity potentially harms up to and including everyone, thus IMO an unacceptable risk of keeping the correlation of the outputs and inputs (the anonymity mix) of a mixing transaction on the block chain.