One would think that the multitude of already established terms for this symbol would, if anything, vindicate the use of the term "hashtag". It demonstrates that there is already a historical precedent for contextual consideration when naming the # symbol.
#tag is a hash, followed by a tag, together constituting a hashtag: lit. A tag denoted (in a snippet of plain text) by its preceding hash symbol :)
# alone lacks the tag to make it a hashtag...
hash -> hashtag hardly seems like a meaningful change