it's pretty obvious when Talon is going for a cutthroat
That just means you're a bad Talon player.
syndra's stun is not limited to 700 range
It is if you actually want to hit it in lane. Thing is, you could set up your QE combo at 1349 range(which is the max range of her stun if you push a ball from 699 range.) but there's a reason why syndras never use that combo in lane vs assassins. If you miss you are FUCKED. You are completely boned. This is why most high elo or pro syndras rarely use their stun in lane and only use it for ganks or just for the knockback when someone is too close.
Also, your passive only procs when you max a certain spell. You won't have that enhanced width that helps you hit long range stuns till lvl 12(unless you're maxing E first for whatever reason). And the pre-12 projectile width is actually pretty crap.
Ahri is a ridiculously easy matchup
In gold, maybe. Ahri is probably the best midlaner atm and Talon should never be allowed to get into range post 6 without taking a shitton of damage in the face. Pre nerf or post nerf. Ahri is just too powerful atm, but that's not really the point here.
low projectile speed
It's 1450. That's the same speed as Elise's stun. Is that really "slow"? Wonder why Elise was dominating in pick comps for so long then, guess the pros are just too slow.
The silence was 1 second, what are you going to do to a tank in one second?? Are you gonna delete their carry behind them in 1 second while you are standing next to the tank with only your w and ult for gapclosing?
Umm, yes? That's what all the good Talons were doing. E near the carry(on the support or the biggest CC threat) WR, flash while stealthed, QRAA(you can cancel his Q animation with ult and still get the damage, it also resets your AA timer). And because you buffer your autoattacks they'll connect even if they flash or dash away.
ess damage than other burst assassins
Sure, Zed deals more damage theoretically, but his damage can actually be negated by zhonya since it's a dat effect. Also, zed doesn't have a silence that would stop the enemy from using his zhonya's.
And Yasuo's isn't an assassin, he's a melee carry. He does more sustained damage but can't explode someone in literally 0.5 seconds.
Riven doesn't have quite as much damage in 0.5 seconds but she has a stun and a knockup so it's fair.
Fizz deals about the same damage, but he needs to wait 2 seconds for his fish.
Leblanc does just about the same thing. 100-0 the enemy carry in under a second, that's why they removed her silence too.
The meta shifting to more mobile adcs
The meta always favoured mobile adcs. The only exception were s3 worlds, but most of the s3 regular season still favoured mobile adcs.
He shouldn't be nerfed just because for a little while the meta favored him. Thats just dumb logic on riot's part.
His winrate is still over 50% so you really shouldn't be acting like riot made him useless. Just learn to go in when their CCs are down, something that other assassin players have been dealing with all this time.