BookBarbarian
Expert Long Rester
Because all of the fictional high and wood elves (who are naturally dexterous) who do not have martial weapon training and also all of the fictional rogues who cannot sneak attack with non-finesse melee weapons are trained in the use of this fictional weapon. So a solution to the issue of fictional Longswords is to make the fiction of them consistent and sensible with the fiction of the non martially trained elves and the class of rogues.
I think we've had some good posts in the thread that would explain why Elven society highly values training with Longswords, even when they are naturally dexterous.
I also think it's perfectly fine to let Rogues sneak attack with Long-swords in fact I would have preferred that they can sneak attack with any weapon that a Rogue is naturally proficient in. 1d10 just isn't much of a bump.
I do dislike making Longswords finesse, as it further tips the balance away from Strength and further to Dex.