idk I play both mages and Assassins and atm I picked up way more 
again even if I'm usually an Assassin main, simply because mages are way stronger atm imo.
Mages don´t really have kill pressure even when ahead
Not true. They have kill pressure, just not as much as Assassins, because, well because they are Assassins and should have more kill pressure, but being ahead on, lets just say , lets you completely dominate your enemy.
In addition things like are straight up pain to push against. No chance.
Then he gave up a lot of gold to keep up with the push, but heavily delays his power spike.
Nonetheless pushing alone isn´t as usefull because ad caster get free autoattack boost for lasthiting under turret (not to mention additional lasthit assist like zeds passive). For some strange reason ad midlaner tend to have wave control with some items and lv6 every time I have a mage on my team. Must be coincidence.
Because most people simply think "there is an Assassin against me, I have to stay away from him no matter what or he will take 80% of my hp". That's true as long as his abilities aren't on CD. Thing is, Assassins have to use them to outpush mages.
And as mage you just have to stay back and focus on the melees, and when he steps up and uses his spells you can safely step up and clear the back minions. Easy.
Last but not least you never ever follow an assassin as a mage even if one ability is on cooldown. Especially not if there ultimate is up. There is a reason mages are most common in mid. It´s because they are usually very bad at dueling. If you dare to miss a single spell you are dead. Of cause assassins can miss skillshots as well but they have something mages don´t have: better sustain dps via ad scaling autoattacks, lower cooldowns and point and click abilities.
Now this is just some biased BS which isn't true at all or simply neglecting important things.
Ofc you can follow an Assassin if he just used his entire combo on the wave, or depends on the Assassin at least, but you can safely follow a no energy Zed with his W on CD, and for other Assassins goes, if they pushed you in that fast so they can safely roam, on top of you didn't setting up wards in their roam paths, you did something wrong.
And the thing with the "sustain dps" is that they have to be in your face to get it off, and every mage has a cc spell thats now easy to land, then there are things like minion aggro, towers, and many factors you didn't even mention.
The most important thing when laning against an Assassin is not to focus on winning the lane or outtrading him, but you want to simply deny him, meaning wait until he uses his core ability to farm, until then focus on the front minions, then clear the back ones, and when you pushed the wave, don't just stand around and give him or his jungler an free engage on you, but try to do things like getting vision in river, so you easily deny his roams if he manages to outpush you once.
are straight up pain to push against. No chance.
and 
would be top pro-play picks and 
would be a soloq nightmare which isnt true.
