I understand your point, and acknowledge that TLS MitM is a major security threat, but my point is that if you're able to get deep enough into someone's network

I understand your point, and acknowledge that TLS MitM is a major security threat, but my point is that if you're able to get deep enough into someone's network that you're able to intercept traffic between where it is passed to the interceptor and then re-encrypted, most information they could get from a MitM could also be obtained by pillaging the network through direct endpoint and server access. If they install malware on your workstation, they'll grab your admin panel and email credentials with ease; they don't need a MitM to do it. You're right that a TLS MitM is one big way to make it easier for them to pivot and install malware, but there are many other ways they can do it, too, if they're on your internal network. I think the pros of TLS interception outweigh the cons. You are exposing yourself to a little more risk once someone has already breached you, but you could say the same about other security products and procedures. For example, if you gained admin access to an endpoint agent like Carbon Black or Google Rapid Response, you can now deploy malware to every machine in the company. The pros of these endpoint monitoring tools outweigh the cons, though. You just have to architecture and protect them properly.