I dunno. I know plenty of people who might want to work on an Excel spreadsheet at home over the weekend and so might e-mail it to their personal e-mail account. They would almost certainly reconsider, however, if it required copying that spreadsheet to a flash drive that they then had to hide in their ass, though.
In that case, it sounds like the device is effective after all.
Suppose you're a college dorm network. Then you can't justify TLS MITM because the risk of your MITM device actively creating a security hole that leads to all the students' bank passwords being stolen is greater than any benefit from centrally monitoring the traffic in that environment.
Suppose you're a highly classified government research lab. Then you can't justify TLS MITM because the bad guys are skilled foreign government agents and you need to isolate the network from the internet.
And there is no happy medium because the risk and cost of having all your TLS-secured data compromised scales with the target value. The higher the target value the higher the risk created by the MITM proxy, all the way up to the point that you can justify isolating the network from the internet.