I know that a Lux that isn't a threat to burst people really doesn't scare me and I say that as someone who plays mainly bruisers and other melee divers. If all you have to throw against me is your Q, I'm going to murder your adc. You have nowhere near the amount of peel of all the cancerous supports that Riot has released in the past few years and you also don't have the damage to scare me off like a Zyra and Brand does. You may think you're doing a supports job just fine as Lux. You know what I think as a champ that has to consistently dive past or through supports? I think Lux is one of the worst "supports" you can pick while still technically qualifying as a support. Your Q on a 7 second CD at max CDR is not going to deter me to nearly the degree that a Braum, Thresh, Lulu, etc. does, and your damage will not deter me to nearly the degree that a Brand or Zyra would. Nothing about support Lux scares me as a bruiser/juggernaut/melee carry main. Nothing. I'm just going to lock in Yasuo, laugh, and 1v2 you and your adc effortlessly as soon as I get a single item.
I literally have this same exact argument with a friend of mine who is a Nami main. He insists Nami is the best support in the game and has for years. I consistently tell him that, as someone who mains champs intended to kill adcs, Nami is one of the easiest supports in the game to outplay. The only thing scary about Nami as a diver or assassin is her exhaust and that doesn't even have anything to do with the Nami. Now take Nami, multiply her uselessness by 2 and you have support Lux. You have 1 long cd spell that can be used as actual peel and your shield is an absolute joke in most situations while your poke and burst threat fall off a cliff after the early game unless you get fed and get lots of AP. What about that sounds good to you? Get out of your own head for a moment and think from the perspective of someone that's playing against a Lux support. Beyond your early poke, why should I give a shit about you? Outside of getting fed, I can't really think of many reasons compared to real supports. You don't deter dives. You don't do tons of indiscriminate AOE damage. You don't poke the enemy into submission. You don't provide powerful utility to your team the way a Soraka does.