As a Lissandra main with almost 500k mastery points on her, I can confirm this. Her passive is only useful until you get a second doran's ring, a blue buff, a catalyst OR a lost chapter. If you run out of mana after that it means you are terrible with Lissandra and should improve your mana management. I think her latest buff was aimed towards low elo players because they tend to spam spells without thinking too much.
Anyways, I can understand why it is hard to give her a new passive. She already has a lot of utility in her kit, and with items her damage is actually quite high. Trust me, there are few things worse when you're playing adc and a Lissandra flashes on you, unleashing her full combo. You'll either end up dead or in the worst case you can't move for 3 seconds and Lissandra's team can finish the job. Messing with her ratios ( or adding a new damage source with a new passive ) would make her oppressing, way more so than LB with sigil of silence ever was.
Riot has to take a different direction if they want to give her a new passive. That mana stacking buff they tested is actually not bad, it would help her to scale into the lategame without making her early and mid game too overpowered. ( except for the on-hit effect thingy, which I think doesn't fit into LIssandra's kit ).