I think it's more that people are wary of how a supporting cast might be used, and how that might detract from Natasha as a focus of the film.

And isn't this a part of the problem? That there are so few female leads in (superhero) films that we're scared that when there's even a possibility of them happening we all have so much riding on them, that one character has to represent so many people which they just can't, and there's worries about other characters... Sure, let's have Clint in Natasha's film. And let's have a Captain Marvel, Wonder Woman, and Ms Marvel film in the same year. And Wasp can join the party when Ant Man happens. And Yelena can be in Natasha's film. Let's have more ladies and characters of colour and disability and diversity.

And then when Natasha gets her own film we'd be just as excited at Clint being in her film as we were at Natasha being in Steve's film.

I have no idea what the short will be, but I am excited! I know I've been missing tonnes of extras by not being blurays but Cap extras.... 2nd or 3rd of August for Guardians for me as it's a group trip. EXCITEMENT :D