Poppy's W needs to work

If a champion is supposed to be able to get past Poppy's W, then they should have a blink. Abilities with an unstoppable dash are frankly broken against her. Sure, it's fine that someone else can't stun them during the move, but Poppy's W is only cc when someone is dashing, and it's on a pretty long cooldown. They have ample time to either be aware that it's been used or wait until it has. And if the top laner they're ganking doesn't have a dash to bait it, too bad. They already counter Poppy themselves by lacking a dash.

It's no argument to say that a champion would lose their main engage if this happened, because bans exist and that's already the case for several champions who aren't unstoppable. Banning Poppy when you want to pick Malphite is an option, and there wouldn't be any other direct counters to Malphite you'd need to be banning instead. Besides, champions like Lee Sin and Riven can't fight Poppy at all, because they always give her warning when they're going to dash and actually can't use half their kits while inside her W.

Poppy's W needs to be an exception to dashes that make champions unstoppable. They can easily ban her, and she hasn't been a popular pick since the CoC removal. There's no excuse to whine that your champion would be countered by her when there are many champions you can pick that aren't effected by her W in any way. Or you could just use your ban. Considering this is the only ability in the game that would be able to stop them, and there are currently no hard counters to any of the champions it would affect, you can easily use your ban on Poppy.