While it is a useful tool for users there are exemptions for the company. For example, if your account details have been requested as evidence in a trial then the company should not delete your account under a GDPR request.

GDPR isn't a blanket deletion. Organizations can still retain personal information about customers for situations where they deem the data is still necessary for the original purpose.

e.g. purchase history, legal agreements, etc.

Not for the original purpose. For legal reasons only and legal reasons are quite narrow. A simple message does not qualify.