Supergirl will go the way of Arrow, perhaps more easily due to being overlooked by Greg Berlanti and co as well as being formulaic. As for the fanbase, when Kon-El shows up there’s a damning chance that some will really latch onto him.
You have younger viewers connecting to him so easily they write a lot about him in their blogs and fanfictions. He ends up overshadowing his brother this way. Then we get people who secretly desire Superman.
Someone pointed out that another problem with female counterparts to male heroes is that they’re uninteresting to people who don’t secretly desire the latter. One shouldn’t be surprised to find sexualised fanart of Supergirl, including a literal genderbent Superman.
If there’s a Superboy some will age him up to be appealing. Some South Park fans do it often. Which further marginalises James Olsen. Which gets compounded not only by his personality change but also Winn who becomes male Felicity.
He’ll be appealing to people who don’t want desire the underaged Kon. Either character, it’s pretty frightening to witness if Kon shows up on Supergirl at all.