Signature allies are some of the strongest cards in the game so to reduce it to just a wild resource is actually pretty big deal. Also, in this specific case, Captain Marvel's best deck imo, is Avengers Leadership which needs Spiderwoman as an Avengers Ally.
That said, the ally is really only important if you care about min-maxing. And if you care about min-maxing then you are probably fine with picking a different hero. If you are a casual group and people are just playing their favorite hero, then your signature ally becoming a resource really isn't going to matter.
It would be nice to see a replacement card because otherwise, I would rather pick a different hero than play the 2 together but that doesn't really bother me and I really can't see them doing it.