I don't think you understand the difference between an auto attack reset and an animation cancel. The former is an attack timer reset, allowing a manual auto based ability to 'reset' your auto animation, IE
q. An animation cancel is cutting off an animation, whether that is an auto or ability animation, by using specific command input to speed up the execution of said animation.
for example can cancel both his auto and ability animations by chaining them together carefully. That is why he can get away with much less attack speed than other adcs, albeit at the price of some scaling power.
Lots of melee champions that build Tiamat have animation canceling effects as well
I will correct your use of "animation cancelling" with "auto reset" to compare it to your item suggestion. Most build it for the other benefits I listed, which was exactly my point. People don't just get items for an auto reset.
Most of the melee champions that even buy
items do it for stats, wave clear, and the animation cancels (that have been mostly removed I believe), not for the aa reset
The only champions that I recall that have animation cancels within their kits and build tiamat are
which is tied to his ult and
who uses it for her ult, because her other cancels don't work on ult animation I believe (I am much less experienced with Riven than Renekton admittedly).
This has power because it works in tandem with champions who work in a hit count system. Using a basic attack, then an ability to cancel part of their attack animation, then a damage-empowered basic attack, followed by what amounts to an attack reset.
When exactly do you think this would fit into builds and on what champions? It sounds like a trading item, but no adc would survive laning building this. Building it second would inhibit a powerspike adc incredibly hard and hurt scaling, making it a "always sub optimal" choice. Maybe Jhin can abuse it, but the keyword is abuse and even then I think he has better items. Ezreal doesn't want a worse triforce, Corki would delay his scaling and powerspike, Kog'Maw can't use it, Vayne can use it at the price of an even worse laning phase and delayed scaling. Kai'sa can't get q evolve with it to be able to wave clear, Varus eats the trade offs like Vayne, Ashe has a reset built in, MF can't get it in laning phase and has low base ad anyway. Am I missing anyone?