People play Tryndamere wrong.
They play him like they're trying to stomp Bronze/Iron elo. They never consider counterplay or objectives beyond splitting or teamfights or anything; they just cheese cheese cheese in lane with a Conqueror build and hope for the best. It's the equivalent of picking all cheese runes on any top laner without a gameplan past the 10 minute mark other than hoping to annoy the other team to death.
The problem with PLAYING Conqueror Tryndamere (even when fed early) is that his opponent has a ticket out of top lane: anyone remotely useful coming and helping him nuke Tryndamere and then kite/cc him to death every time he's pushing - but Tryndamere at that point has no escape. Even if he's mega-fed using said cheese build, after doing nothing but pushing top lane in for 20+ minutes the counter to him is obvious and very accessible: focus him for just a little while, cc him/kite him, then finish him off with Ignite or ranged attacks. He's so easy for 2+ non-feeding champions to deal with and play around unless he builds for lategame and shows up to teamfights instead of cheesing early and doing nothing but push.
Scaling runes that are FANTASTIC on Tryndamere, particularly outside of lane:
Lethal Tempo - Bonkers mid/lategame
Hail of Blades - Good all game, even better at cheesing than Conqueror. Why does no one run this?
Legend:Tenacity/Unflinching - ...why do people value a bit more AS over free tenacity on THE premiere "just CC him" champion!?
Revitalize + Barrier/Triumph/PD - Live much longer after ulting, potentially be able to still fight while ult is on cd
Approach Velocity - Catch someone with W or a friendly tank gets CC'ed initiating and it's a free mini-Ghost with no cooldown! Abuse the whopping 1000 range.
Items that make him pop off when finished in laning phase:
IE - Duh
ER - Extremely valuable on Tryn; have ult up every fight and E more than once no matter what
Boots of Swiftness - Same as Unflinching/Legend:Tenacity
Titanic - Sounds silly, right? But wow this item can put in work on Tryndamere, alongside a couple health items like Sterak's. It also makes him seriously beefy in teamfights - particularly coupled with maxed W.
Clearly it's not Riot's fault that Tryndamere is boring to play against; it's just ours. After Riot revealing the next 5 champions lined up for reworks over YEARS to come and Tryn being a tier 5 top laner USING the same old hashed bad cheese strats, despite all these powerful runes and items at his disposal, it's apparent that very few macro "tryhard" Tryn players that actually care about winning exist. Only wishy-washy FotM players are using him for the one brainless strategy to gain (what they hope is) free Elo against bad top laners.