AFAIK cryptography is never "safe" it has many weaknesses.

1. the vault is only as secure as the key

in theory you never need to encrypt you private keys, just keep them in a place where others can not see it or access it.

2. all encryption can be broken with enough time AFAIK

3. to combat QC now, simply increase the encryption now to unrealistic heights. then you have time to reimplement the needed security once you understand the problem.

to find a private key from a public key would still probably take a while, my guess would be at best, a few days.

4. alternate chains will emerge and probably try to address these problems

and lets not forget the most important thing.

i have yet to see any real benchmark of QC doing real work, and i don't see that happening in the next 5 years.