That's still enough for people to unite behind his cause should it ever be needed, I think. Plus, Jon would likely support his claim as well because of how reluctant he is to take up the reigns of leadership himself. Although a more logical explanation for Gendry's reintroduction to the story is that he's become an incredibly popular loose end since season three. The writers can bring him back to avoid any post-story complications as to his whereabouts, and then have him do a few badass things with that hammer while smiling enthusiastically to please the viewers. If it comes up in season eight that he should have been king after Robert's death then they'll put that in - if not, they kill him off and get an emotional death out of him. Everyone goes home happy.