They could have easily killed Ezra, yeah, but it wouldn't necessarily be a better story. Plus, I think what makes Luke special isn't so much that he's the only one who can use the force at the time of the OT, it's the fact that he's the only one who can save Vader. Luke beat the Empire because he loved his father too much to give into Palpatine's **** about hatred and the dark side. Obi Wan couldn't do it and Ahsoka couldn't do it, it was only Luke throwing away his weapon and proudly proclaiming himself to be his father's son. Some other guy outside the galaxy with no possible way of returning on his own isn't going to change that.