The core market for the feature is schools, and that group is generally okay with the filter being conservative and low-granularity (read: little thought required for configuration)

Users: Somewhere in this nation, there is a high school administrator who consents to students learning about politics. There might even be more than one.

Another company could pick up the slack. Hell, PragerU could do it. Nothing stopping them from brokering their own ad deals to run prerolls on video they host with their own infrastructure. A few schools might allow access, and many will just black-hole prageru.com in their firewalls.

At Google scale, one has to weigh the risk of false-positive and the risk of false-negative on a low-granularity feature like this, and the risk of error due to misconfiguration being blamed upon YouTube if it's turned into a high-granularity feature. Because the failure mode for YouTube if they false-negative something that is visible to students who should not have seen it (and generate a negative press cycle for themselves) is that schools choose to black-hole youtube.com instead of bothering with Restricted Mode at all. That's what was happening before they added the feature, and it's the reason Restricted Mode exists.