Gameplay updates are not a matter of power or matchups. If a champion is overpowered or underpowered, they need changes to their balance, rather than to how they work. Similarly, champions with too many good or bad matchups may otherwise have perfectly healthy gameplay, and so may not need much more than changes to their early game power (if they even need that much).
Ultimately, the problem with Tryndamere isn't that he has too many good matchups or is overpowered in any way, but that his core gameplay is simply not healthy. The whole notion of gaining random crits at level 1, and then gaining even more power off of those random bonuses with the rest of his kit, means a huge amount of Tryndamere's power isn't based on real interaction, and can completely turn a lane in one or the other's favor in a manner that is impossible to accurately predict or prepare for. Even if Trynd were to be nerfed to sub-40% win rates, this problem would still remain, which is why he needs a gameplay update.
Beyond just Tryndamere's reliance on random crit, a mechanic Riot themselves have denounced as unhealthy for years now, the other main problem with his kit is that it only functions in the least interesting circumstances: Trynd is infamous for being a splitpushing machine, because his high stats and waveclear let him quickly go down a lane, and his ult means he can usually leave safely and even kill lone opponents who go out to fight him. He is also, however, terrible in teamfights, because he's far too easy to kite and lock down, and doesn't really contribute much when he isn't two-shotting squishies. Effectively, even if Trynd's entire crit system were reworked, he'd still be a dysfunctional and mostly uninteractive champion, and so needs an update to his kit that encourages him to fight with his team, and not just isolate himself to top lane the whole time.