This is true in situations where there is no social contract for the event at hand, like emergencies and other one-off situations. It doesn't apply where there is a social contract. Everybody is responsible for obeying the traffic laws, for example. And everybody can be responsible for maintaining conceptual integrity.
I think it applies here. The problem which conceptual integrity is that short-term incentives oppose it. A developer gets personal benefits for shortcuts (Yay, a 10x rockstar developer!) while the costs are shared by all. A classic tragedy of the commons situation.
Maybe you assume that random developers will speak up, point out an inconsistency, and everybody agrees to make it consistent from now on.
In a small team it might work that way, but "system design" implies a larger community to me.
Consensus is hard.
The whole point of responsibility is that blame and praise is focused. A focus on everyone is no focus at all.
> Everybody is responsible for obeying the traffic laws, for example.
Well, we do have traffic police and courts and lawyers.
Yes, everyone should follow the rules. But the point of having an individual responsible for conceptual integrity does not mean, no one else should care and do whatever they want, but that there is, if needed, "police and judges" in place...
But there is another point: the fewer people the easier it is to have a consensus about what the conceptual integrity entails.
If you have a single judge, you are more likely to expect consistent rulings, so to speak.