Let me ask you something. How many Syndra's do you see running around each game? Ok, now let me ask you this. How many Syndra's will you see after this buff? It's like if Galio got true damage added to his maxed resolute smite. You still won't see a lot of him even though his R can 100-0 an adc.
Now let's see. Press R to delete carry. Late game with 6 or 7 spheres maybe. Let's not forget that she needs setup unlike someone like Veigar or Annie who can just pop ult and delete you as well. Not to mention her ult range SUCKS pre-max rank. If you get deleted as an Adc by a Syndra ult, you got outplayed. You have to LET her walk up to you and press her ult. By then she should have been deleted because you could have shot her with 4 to 5 basics which is more than enough to kill her.
Also, does the phrase "immobile mage" mean anything to you. You can get deleted by Ekko, and Fizz and Riven and theyre all fine, yet they all have mobility. When Syndra gets a mid-game buff, she suddenly becomes overpowered. Watch her blow up when your team's master yi or rengar jumps her.
Now. AOE stun. We must be thinking of a different champion, cause the Syndra I play, has a cone knockback with a line stun with a sphere applied. Granted, you can get off a 4 or 5 person stun if the enemy team is stupid enough and let's you walk right up to them and let's you get all your spheres in place. No, she'll be deleted by the team her set up is complete.
15% true damage at max rank. This will only show against tanks. How many times are you playing Zed or Viktor mid against Syndra and think. I should get a crap ton of magic resist! You don't. You'll only see a slight increase of damage and more against heavy tanks. And we're talking about mid game power. She doesn't just start out with 15%, she has to earn it, granted you haven't already killed her a million times because she's as easy to camp as it is to count to three.
And, insane range? Hmm, from what? Her W. Syndra is close range and must be close range to get everything in place. Her stun is close range. Her ult is close range. Where is this delete carry from across map idea coming from?
She's not overpowered, she'll just see maybe a 5% increase in play.