But there is no way to build against %health true damage.
LoL is a game with strategy elements though: not being able to use the same tactic for everything (building defense in this case) is part of what forces players to deal with situations intelligently over resorting to just trying to muscle your way through every problem with the same mindset.
There are plenty of cases where other strategies are rendered useless or sub-optimal due to game mechanics as well.
Knockups, Knockbacks, Pulls, Flings, and Slowing fields are completely unaffected by Tenacity/QSS/Cleanse.
Mobility cannot be used to evade a wide range of point-click skills.
You cannot damage opponents that are invulnerable, in stasis, reviving with GA.
Wind Wall stops projectiles.
100% dodge chance evades 100% of auto-attack damage.
Spell-shields prevent insta-gib 100%-->0% combos.
Mobility skills can prevent certain champions from ever really overextending.
Range can be completely negated and rendered useless by mobility in a 1v1.
Stuff like this is what leads to dynamic game-play, where strategies and tactics must change to adapt to what's going on - rather than having a cemented meta play-style that's always the best.
Also - it's not like Fiora's new ult damage lacks counter-play. She needs to dance around her target, auto-attacking them from 4 specific angels in melee range.
Do you know how difficult that is? In a 1v1 that seems difficult if the other person notices what's going on with the big flashing targets. In a team fight? You're melee-range Draaaaven with telegraphed points in significantly easier locations to take advantage of.
I mean - really think about what Fiora has to go through to apply that %max HP true damage.
Almost any other stratagy besides building defense is going to be able to prevent her from applying it: including listing lazily to the left.