If you are uncomfortable with deleting the folders off a file share immediately after migrating to SharePoint, simply mark the folders as read only to prevent users from modifying the files. Once you've become comfortable with that, you can then revoke everyone's access to the folders and then delete it at a further date.
If the concern is hoarding, then you need to communicate a clear migration process. "The file share will be migrated to SharePoint on XX and at that point the share will be read only. This will allow for XX week(s) of data validation by both IT and business users. On XX-XX-XXXX, the data will be deleted from the file share."
We have also used written sign-offs of the data by the owners that they've reviewed the SharePoint site and confirm they have what they need. If they sign off early, we kill it early. If they don't sign off by the deadline, then we can determine what happens (like revoking all access versus deleting).