removal of actual responses for example

I am not a specialist on GDPR because it doesn't touch me this much, my targets are generally not in or from the EU. But of course it is very interesting for any market researcher. We all should already have had a high standard of data protection (at least in theory) before the GDPR.

However, I am wondering if deleting all answers to a questionnaire is actually necessary or if it is only necessary to delete personal data? Because what about anonymous surveys, where the results are not connectable with the responses anyway? You can't delete the survey responses there, because you don't know who they are from. If you delete the personal data from a non anonymous survey it is basically the same effect, isn't it?