From HTML WG Wiki
Related Solutions Do Not Negate the Need for longdesc
longdesc is being used properly today, is a valid HTML 4 attribute, is a viable choice that is supported by the majority of screen reading software, and when used properly, it provides a needed and useful solution to real problems.
The existence of related solutions does not negate the need for HTML5 to offer longdesc as a tool for authors. It provides them a unique tool when they determine it is needed. In many circumstances it is the right tool for the job. For instance Gail, Wylie, Toby, Clara, Naomi, Arvid, and Bill, Bob and Carl, as well as groups large and small all choose it. As do many others.
Stripping it from the author's toolbox would be a disservice.