Hmm, well I appreciate you looking at my idea. Wanted support, but attention works too.
Why do you feel all other boots would become obsolete? Surely everyone wouldn't pick this up? In fact, I doubt most would. A DPS wouldn't typically get this as you have better selections. Maybe a support might.
Also, it doesn't need to be boots. I want to focus on the active. An ability to decrease mobility from spells and abilities for a period of time in a specific area. If the cooldown was high enough and the cost was high enough, it would inspire one purchase for the team. Probably a tank, or a few specific supports. (At least in high ranked games.)
I feel there aren't sufficient counters to high mobility champions. You have exhaust, but a flash sufficiently separates you from them to the point where it is almost resolved before they reach you.
Brain storm with me Astaare (and others)
|it would either disable or remove the effect of flash, ghost, and abilities that allowed someone to move faster than their movement speed|
As for your actual idea... I like it. J4 players would probably love it (no more flashing out of his ult).
Problems I see with it: Well, the price of this item would probably be fairly high relative to the other stats it provides, since that active is so powerful. The cooldown would have to be pretty high, or the duration very short, since otherwise it would feel like a single item just made the ghost/flash/shurelia's/etc that I've got feel like a complete waste (it'd feel pretty bad if your lane opponent could prevent you from flashing away during every single gank). Basically what you already said. This active probably would need to be on an item other than boots, so you'd have to give up a slot for it (think about it like Quicksilver Sash: powerful active, simple recipe, not a ton of stats).