Sorting by first post is useful. For example. I have a course issues and suggestions forum, and it is absolutely useful to sort by first post date since that tends to match the point in the course when the issue occurred better than the date of the last reply. Similarly, I want students who enrol late to read announcements that were made before they enrolled. Sorting by 'last post' is not good enough since replies to posts, which frequently are not relevant to later enrolling students, alter the sort order.

There is code in the linked issue that might solve the problem that does not appear to have been  incorporated into moodle's codebase. 

It should be easy to implement. All that needs to be done is to add the creation date field to the forum display code. The field exists. You can see it when you export the forum posts.