I managed to avoid all the spoilers last thread so maybe some of you guys already addressed this, but I just finish the story and it seemed pretty clear that Miyupi managed to find SiL again at the end, and that they're not dead or anything like that. Just that they both retire from the racing world to be together.
Having them actually be dead would also feel pretty out of place for an Uma story, then again, nothing about this campaign is like a normal Uma story. Including that final line.
Overall though, this campaign is amazing. It's not even an "I can fix her", it's "we're going down together." Really didn't expect that from this game. I don't know why, but the ending really reminded me of this:
https://www.youtube.com/watch?v=cPNyNUO6JMo