In few words: less work, doubts and backward compatibility.
Personally I strongly support the proposal to write a description for each tag: it will avoid loss of time later when it will be needed to retag the questions and speed up the process to select a tag for the OP.
I think that it can be proper to order the list by the number of new questions in the last 12 months.
At the same time I would avoid to delete tags used more than really few times: instead it's possible to make them synonym of the master tag and eventually to change the excerpt of the deprecated one.
This because when synonyms it happens that
Tag synonyms are replaced by the respective master tag on almost every occasion, e.g., when you search for a tag, try to apply a tag to a question or follow a link to the tag.
but it will ensure that, for example, each research made once with the deprecated tag will give again in its results the posts that were reported before (in addition to all the new ones relative to the master tag).
All uses of the synonym tag for any given master tag are automatically converted to the master tag.
Moreover did we consider what will it happens to the followers of a specific tag when it is deleted?
Ps> Sorry for late joining