Since TLS Interception is critical for some places to achieve their duties (either auditing in banking, or content filtering in schools) we will always have some kind of legit use for MITM CA boxes.
It gets slightly worse if you look at how AntiVirus generally installs CA's to do malware content filtering. Cloudflare publishes stats on what % of traffic they think has been MITM'd: https://malcolm.cloudflare.com/
In Poland where I live, parental control basically doesn't exist. Children routinely watch movies, play games and visit websites that they're theoretically not supposed to visit. Parents know about it and don't really see an issue with it. I've seen several boarding school networks, none of them had any filtering whatsoever. The only filters I've seen were on computers in IT classrooms, but nobody uses those for non class-related activities anyway. As far as I know, we don't even have a law banning selling x-rated content to minors.
I worked on a transparent MITM proxy, a proxy that is configured in the firewall not at the application; IE applications were unaware they were using a proxy.
Simply adding the proxies CA key to the system trust store was all that was needed to make all applications, including browsers, to trust the certificates we were manufacturing.
There was no indication besides inspecting the certificate that the certificate was ours. Definitely no calls go check certificate transparency that we could see.