I mean, you'r technically right, but you're assuming perfect conditions. A 5-man Q-aura on a single person? In soloq, no less? That's rarely going to happen. If you can get you're team to cooperate, it's great, but they usually have other priorities in fight.

You also have to factor in the risks involved. Sona, alone, needs to get into 550 basic attack range for full damage. That equals about 650 spell range. In lane, that's horrid. Despite her being able to do it from behind minions, plenty of champs can punish her for getting that close.

Leona was a top pick against Annie (when she was strong) because she can withstand damage like that and punish Annie for getting so close. Leo can still do that with Sona. Alistar can also engage when Sona tries to poke. Thresh can flash-flay, and blitz can flash-knock up. Taric can stun. If they get close she's either dead, close to dead, or without ult and/or flash.

Many that can't punish her hard for it have no reason to let her get so close until they engage. Morgana, Karma, Vel'koz, Zyra. None of them have a reason to get within 650 range when Sona has her powercord up. Soraka can mostly stand back, if she has HP regen runes and/or coin.

And of course, the ADC may have problems against her. She is able to poke them when they go to farm, but she needs to gets her powercord up for that kind damage. In lane that means using her Q (which will still require her to go within 650 spell range to make the most of it), W (which decimates her mana and isn't always needed), or E (she doesn't want to waste her only source of mobility outside of flash). That still involves her getting near her enemies minions, where you can punish her. Even the minions will help return the damage.

If she lives, she can heal up. But, her repeated use of her heal decimates her mana, and it takes quite a bit of time to fully heal. This can be mitigated with potions, but the enemy support can also buy potions.

When the teams start grouping, no more poking with powercord. Unless someone is seriously out of position, to many champs can take advantage of it. Best save it for a teamfight, where W or E powercord are going to be more useful. Those to are really underrated. Your team shouldn't need damage (although sometimes it is best). Seriously, 20% damage off of that assassin or ADC for 3 seconds? 40% slow for 2 seconds? Versus somewhere around 224 (+40% AP)?

TL;DR: Yeah, she has great damage for a support, but there's great risk involved.