that sounds like a dota2 item
Are you aware that in League, the longest stun possible is by Ashe's Crystal Arrow, maximum of 3.5 seconds? In DotA, there are a lot more abilities that easily break this cap and even penetrate states of immunity.
why the fuck should i buy a useless late game slot for 20% slow on 1 second?
Emphasis on the useless here, because this statement is ridiculous.
Champions that aren't designed to have slows in their kit, suddenly have it and that's OK?
40% was a tad too much, don't you agree? Especially when Rylai's came from "no one's item" to "every mage's item" in just a patch. And it wasn't the slow that was the only problem, it was the HP and AP that it brought along with it, as HP is a universal stat that makes a target harder to kill.
Mages can usually purchase HP but in their items, it always takes time to ramp up.
Rylai's is just a stick of HP, with super stats and a passive attached to it. And when every mage suddenly starts buying it, you know you have a problem. Refer to Frost Queen's Claim problem, when it was too good to be true and every mid laner bought it, profiting more than supports could have.