Question: Is it possible to be able to delete a member without deleting his/her comments for posterity purposes such as retaining it for forums and the like?

Answer: 
If you're comfortable with SQL, you can update the database to *change* the member\_id on the comments to one that's not being deleted. So, you could create a 'past member' member, and then update the comments to use the member\_id of the 'past member'. This will disconnect those comments from the members you want to delete.

Other options:

Save a backup of the database, and then delete the member and their comments. If you ever want to see the old comments, run a copy of the site against the older database.

Don't delete the members at all, but move them into a member group that has no privileges.
To answer your direct question, it is not possible to delete a member and keep their comments using EE's native process of deleting members. Deleting a member from the control panel will delete the comments. I believe you can elect to have entries/comments moved to a different member.