Pfff... seriously dude? 400 stacks is way too much and it's really unrealistic.
What do you propose as a better number? 300? 350? What is the margin?
Specifically in this situation, your match history shows that you've played a lot of Cass in ranked (I haven't played her a lot in ranked, but I have played her a lot in normals, so I'm curious to know more about what you're seeing in competitive mid play with her). Cass obviously excels at defeating a very specific type of mid laner, and there are a lot of situations when it's not optimal to pick her (against characters like Fizz, Ahri, and Zed, for instance--though I might play her vs. Fizz). The same thing is true about Nasus: he does well with a specific type of team composition that can fend for itself while he finds opportunities to split push while also farming.
As far as the games lasting 30 minutes thing, I could actually see this being a problem. But, I feel like it's almost the responsibility of you and your team to say, "Okay -- we're going to try a strategy where we defend our turrets for as long as possible, and draw the game out as long as possible, so that we can allow our star hypercarries to farm and become strong." This is probably unrealistic in solo queue, but in team competitive play it's a viable strategy, and it's used in the LCS all the time.
All they have to do, is to waveclear/shove the lane and go roaming, bam, you will never be able to reach 400 stacks within reasonable time, since you are also forgetting one important factor; rotations and grouping.
Yeah but, any mid laner can wave clear, shove the lane, and go roam. As in -- you can do that, too. You can be aggressive. I feel like Cass can really push her lane and actually lock her opponent at their tower for a good while. She is fantastic at punishing people right behind their turret, which helps you apply stacks to your passive as well. Obviously a good jungler is going to make their way around in good time, but so what? Presumably you've got the lane warded and you can just go back to safely farming.
Now that you mention it, though, the passive really does make it a bad idea to roam--it's a lot of wasted, valuable time. That's probably not a good thing.
Late game is not a certainty anymore, especially if you just "afk farm" all day that passive, having zero impact to the game and effectively making the game 4vs5 for your team.
I understand where you're coming from here. I hate playing with Nasus sometimes, especially when he just goes away forever and refuses to interact with the team. But I think one interesting thing about the passive now, that makes her different, is that when Cass participates in team skirmishes, even if they're not full 5v5s, she applies stacks very efficiently by damaging enemies. Nasus cannot q minions during a team fight, for instance.
So Cass can join in on rotations and can roam if she wants to, but she has to be very careful to make sure she's in a position to do damage as a result, otherwise there's no reward for the risk.
You would have to fight for CS with your own teammates ffs.
This is pretty much my largest qualm with passives like Veigar's and Nasus's. I like them, and I think they're interesting and provide depth, but it does create a sense of priority among all the gold that flows on the map. Who gets it? I think if you played with a team that was like, "we're going to do everything we can to get Cass fed and to get her snowballing," then this might be less of a problem. If you're on a roll at 20 minutes and you have 300 stacks, you're probably going to be owning everyone anyhow.