Tryndamere lacks of tank killing is compensated by his insane hilarious damage, the dude is one of the only champion in the game that gets a crit steroid AND an AD steroid at the same time
a crit from IE trynd will remove half of your hp with up to 900 damage unlike any other character, while the dude has a great attack speed
additionally like you said, the dude has a 5 sec immortality, he can build last whisper he doesn't care if he is going to die
Jax is outdated and need a hybrid shred like
who got it in her rework, honestly jax is the only skirmisher that tickles tanks
Ekko is meh, not a real skirmisher. he's just an assassin that needs to AA a target few time with his W passive to get them dying. they had no other classes to put him in but skirmisher, but i don't think he's a real skirmisher
Riven on the other hand was not designed to be a skirmisher, originally she fell off in late game and was supposed to be a bruiser
Then black cleaver was reworked and since then she doesn't fall off and her win rate keeps increasing with the duration of the game
MORAL OF THE STORY
those dudes cannot teamfight if they build armor pen and become squishier
this board on a design level just does not understand what is the drawback of having 100 range instead of 500