Except Trynd still takes damage during that ult, so that when it ends if he gets sneezed on he dies. So it's not like a Zhonya's. And last time I checked, Tryndamere's auto's don't cleave with the radius of dragon pit like a Fiddles ult would.
Tryndamere is a supreme duelist. All of Trynd's damage output besides his spin is single target(Barring Hydra, but good Trynd's usually prefer stattik shiv and Botrk to that) and locked into his autos. So, no surprise when he kills a burst mage like Syndra, Veigar, or Cass. If he wasn't able to do that, then he'd be rather useless besides splitpushing(And even that is successful thanks to him being able to 1v1 very easily.) Point is: Don't try and duel a dueling champion as a mage. Would you try and kill a Jax as Veigar? A Nasus as Syndra? An Olaf or Riven as Cass? Unless you're really mechanically skilled and equally fed if not more so, you lose all those fights. Same with Trynd.
Sorry, but Leona and Vayne dying to a Tryndamere alone? Either both were low health, or the Vayne was very underfed or bad. They both have loads of kiting and CC to lock down a Tryndamere from 100-0, and even more after he pops his ult. Hell, even Vayne alone can destroy a Trynd if she's good enough.
You speak like Tryndamere is some unkillable god and has his ult whenever he needs it. Sorry if this goes against what you believe, but that simply isn't true. Trynd's a great 1v1'er. As soon as that turns into a 2v1 with any opponents with CC and equal gold, he dies or runs away with his tail tucked between his legs.
If you nerf or take Tryndamere's ult away, you ruin what is basically Tryndamere. Trynd has his clear strengths and appropriate counterplay. Don't get mad if you refuse to see and use them.