Eh. Even Youkai aren't immortal. They die eventually, it's just not known how long it takes.
Marisa could, if she wanted to, become a magician-type youkai like Alice and Patchouli. She just has to discover the right spell, and that's all it took.
The only true immortals in Gensokyo are Kaguya and Mokou (and possibly Eirin), and the ones who operate on the other side of the life/death boundary (Yuyuko, and likely Shikieiki and Komachi).
I always thought Yukari would be a immortal too, or if she is not normally an immortal, she probably would know how to constantly dodge death to become a practical immortal.
This flash brought tears to my eyes.. I have always imagined Alice and Marisa's relationship to go this way, everything is lovey dovey, until Marisa ages and grows old, but Alice does not.
Marisa becoming a magician isn't really a question of if, but when, if we're going into life after the games. Those two will actually have a similar life span, unlike some of the other common pairings.