That's realistic, sad and moving. Risa might come back but probably not as she is someone who wanders but always going forward to her next adventure.
She gives misaka some life lessons. Although I don't agree with one of them. Don't hold on to the past or it will stifle your abilty to enjoy the present and grow a better future. And you don't need other people to find meaning and happiness.
I'm not a fan of the latter. Friends are not necessary in a way. You won't die of malfriendishment, but you will struggle to find a peaceful emotional state if you're completely alone. It's typically not healthy. If misaka is someone capable of going it alone then that's great.
it's sweet that she was able to meet someone that could give her the strength and hope she needed to free herself from her misery. I lament that the ending is so bitter but it feels all the more real and poweful for it. I've met lots of people that came and went with time. Pleasant experiences that were never meant to last longer than they did. It's relatable.
I would love to see misaka when she finds the happiness that she deserves.