Originally Posted by Tjlep
Tanks are a different story as you already know. They give up a lot of damage output and the advantage of ranged abilities(in most cases) for their defensive stats. This allows them to live through a fair amount of damage. Similarly with Lux, you trade a lot of your late game relevance for early game power. There is going to be a point in the game where you start becoming less effective.
AFAIK, the only technically ranged tanks would be Kayle and Cho'Gath. They (along with Nidalee) are the only ranged-autoattack-capable champions that get magic resist on level up: all other ranged champions keep their fixed base 30 from level 1 to 18.
As for relevance, technically anyone that's not a carry (AD or AP) is going to surrender some of their importance over to their respective carries: that's part of the design. Not that a support becomes less important, or that a support needs to ensure the game doesn't last long. Rather, it's just that their role will shift: early-game a support is fearsome and dangerous, while their paired carry less so, and it's the support's job to ensure kills for the carry and protect them, while later on it's the carry's job (well, one of them) to protect the support by nuking anything that tries to touch them, while the support keeps the field tilted in their favor.
Lux, while not QUITE making the cut for a full-on AP carry, (not all AP mids are carries, though pretty much all AP carries are mids) is still very dangerous. She may be, in fact, the most capable of all with her zoning powers; she gets TWO AoE CCs, including a highly persistent one; few will willingly walk through her E for fear of it detonating on them. Lastly, the cooldown on her dreaded Final Spark gets low enough that it can help keep the enemy from grouping too tightly out of fear of all getting hit, hence making for a less-dense target easier for the carries to pick apart and focus down.