Kalista's balance of power

I've played Kalista a bit, even though I'm not the best adc, and my friends have played her too. As someone who likes to design champions, I try to look at the new champions that are released and see what riot has done to make them good.

But there was something that felt really glaringly wrong with Kalista - The balance of power completely hangs on her q. You hit someone with their spear - they get chunked. Now you're ready to follow up with that awesome spear-ripping thing, and... it's like a flea breathing on them.

Rend and spear stacking seems to be a much more interesting mechanic than simply hitting someone with a spear, yet it does so much less. The only thing that's useful for it is the slow, which is okay as it is.

The only thing that separates Kalista's Q from the "Q - deal damage, short cooldown" is the mechanic where if you pierce an enemy then all of the spears on them carry over to the next target. But this is hardly rewarding at all, as even if you somehow did manage to get 2 or 3 spears in a minion and then land the Q on a champion behind it, the Q would still completely outdamage the follow up E.

When I saw Kalista's skills, I had imagined it to be a short cooldown Q that did low damage, making use of her passive and allowing her to stack spears through minions, then the E being the thing that brought the damage, like twitch's E but usable with various numbers of spears for different situations.

Seeing other champions' designs and other parts of Kalista's design, I'm usually confident that Riot thinks about each aspect of a champion. So does anyone have any idea what the reason behind this decision was?