alt Hacker NewsClients are supposed to check. For example, Apple requires a varying number of SCTs in order for Safari to trust server certificates. https://support.apple.com/en-us/103214
And yes, it does break MITM use cases, for example on Chrome: https://httptoolkit.com/blog/chrome-android-certificate-tran...
So how does that work with middleboxes? Corporate isn't about to forgo egress security (nor should they).
I don't currently MITM my LAN but my general attitude is that if something won't accept my own root certificate from the store then it's broken, disrespecting my rights, and I want nothing to do with it. Trust decisions are up to me, not some third party.