When It's Spring Again (HF Normal, AU)(Rin, Sakura)(One-shot)

[Kotonoha]

But, then again, when Saber sits around waiting for Shirou to just show up, that's apparently sweet and heartwarming. Mainly because he actually does....

King Arthur waiting in Avalon is more accepted because it's what's supposed to happen anyway (hang around til England needs her blah blah)

Whereas a teenage girl waiting around for her dead highschool crush until she becomes old and dies is more lolwat

[Seika]

But, then again, when Saber sits around waiting for Shirou to just show up, that's apparently sweet and heartwarming. Mainly because he actually does....

I think there's also the fact that there's an element of active participation here on Shirō's end, and the fact that they're both still roughly alive, help. People like the pair of them, so both of them doing something seemingly impossible works; moreover, taking the pair of them, there's that active element - in contrast to the travails Shirō is seems to go through, Sakura essentially sits and waits, which has very little effort implied (I'm happy to admit that I don't think there's an massive difference between Saber and Sakura in this scenario). Also, on a basic level, no matter how much we're told "It requires a miracle", the reunion of two living people seems basically more likely than a person who burned out his mind and body and buried under a collapsing cavern with the stillborn remains of All The World's Evil actually turning up on the doorstep for Sakura. That's a fairly final death without an avatar of the Third Magic there to save you.

Whereas a teenage girl waiting around for her dead highschool crush until she becomes old and dies is more lolwat

Exactly. The pursuit of the 'impossible' also speaks to that part of you that can admire Shirō despite the madness of his ideal and which so much of F/SN draws on, its conceit fundamentally being that of a mythological mega-crossover fanfic. For some people (judging by the apparent popularity, most, but I wouldn't care to definitively say that given all the other factors), that continuation of the theme* works better than Sakura being so attached to Shirō that she pines for him - and, it's hinted, does very little else - for her entire life. I mean, HF Normal's my favourite end, and I can certainly see where they're coming from.

*swear I didn't realise what I'd done there until I typed it

[Altima of the Gates]

For some people (judging by the apparent popularity, most, but I wouldn't care to definitively say that given all the other factors), that continuation of the theme* works better than Sakura being so attached to Shirō that she pines for him - and, it's hinted, does very little else - for her entire life. I mean, HF Normal's my favourite end, and I can certainly see where they're coming from.

Depends on who you talk to honestly. And even then, she seemed to live her life, but she was attached to the house.
Plus, she did keep in touch with Rin, and even taught her grandchild. Even then though, honestly, it'd take more than just Rin being there for her issues to be sorted out. Her life is the kinda stuff you'd definitely need to seek help from a professional to help sort out. But unfortunately, magi don't seem to have those.

To quantify it as just "teenage girl pines for her boyfriend" is pretty much missing the point.