Heaven forbid they have a non-optimal story based approach to the races. Not everything has to be mechanically advantageous. Elves and longswords have gone back at least as far as 1e. It's a DnD-ism.
Yes but back then it made sense, now there is no reason a longsword would be a traditional weapon of elves, every elf for the most part would have a higher dexterity than strength and so the standard weapon should be a finesse weapon.
Just do as they did in 3e and the above poster suggested and refluff the rapier to an elf blade and give them proficiency in that.
And everything should be mechanically advantageous to someone and longsword proficiency for elves doesn't seem like it would fit any realistic character build. Ohh oh I said build heavens will shatter people optimize build and min/max characters summon the gods.