the thing is that
a)mage indeed walks up to you, not blinks to you but walks, which gives you time to react
b)mage usualy tosses skillshots or aoe from range , which gives you some time to react
c)mage is stopped by your frontline giving your squishies some space
assasins need help thats a fact, but a buff is the worst thing that could happen
what assasins need is to create their gameplay pattern from scratch, right now we have 2 types of assasins, both insanely toxic
one type is a power assasin, he waits for a moment and decimates your team, examples are yasuo and katarina
second type is the single target assasin who just goes in removes your carry and gets out, all in the blink of an eye examples are zed and talon or leBlanc
what is the problem here is their binary design of "wait for a good moment, and safely destroy either one guy or entire team without giving the enemy a chance at retaliation"
as far as assasins represent such a playstyle riot will hunt them