Marisa determinates to follow her dream, of reaching the star.
She knew that human couldn't do that, and if they do, they will be dead.
All other people either tried to stop Marisa, or just accept that they can't stop Marisa cause... she's Marisa.
The end could be anything, Marisa go for star and would come back, or she's gone... forever.
That's just my understanding
I don't necessarily see why she would die, and it seems a little silly that someone as ambitious as her would recognize the lack of need to become a youkai but head directly to her death.
There's nothing really complicated to understand here. Marisa is going to space(?), Reimu doesn't want to be alone and there's no telling when exactly Marisa will get back, sappy romantic feels, blah blah blah. Very sweet. Pretty heart-wrenching, too. Think I teared up a little myself seeing Reimu cry like that, poor girl.
