Not sure why people find this sad. They almost didn't see each other for the last time. But by chance they were blessed with a chance to express their love, which in turn motivated the boy to persevere. I think it's worth being happy that such a thing happened, as opposed to the alternative. Which would be regret in solitude. And after all that, it leaves you with an optimistic ending. It's a good ending. Sure it's not the super happiest ending ever, but I don't think that's a reason to be sad.
"Sexy Onee-san" has (Ero-) Power Level which is Definitely OVER NINE THOUSAND! And yes, she's worth pursuing until you catch up to her. Do Your Best, Akira-kun ... you Lucky Bastard!