Personally, I wouldn’t be surprised if the end of Season 5 features a brief tease giving us a clue to who Vigilante could be.
I can’t think of a time in Arrow where a villain or hero has remained mysterious for an entire season, so whoever it is must be a big reveal.
Going back to the prior statement to EW, Marc Guggenheim’s “probably,” in the quote makes me wonder if a Season 5 reveal is completely out of the question.
That’s definitely disappointing, but not all that surprising.