I guess my toaster is going to hack my printer someday, but at least it won’t get into my properly-secured laptop that makes no assumptions the local network is “safe”.
This way you can ensure you as the developer have full control over your applications' network communication; by requiring client certificates issued by a CA you control, you can assert there is no MITM even if a sysadmin, user, or malware tries to install a proxy root CA on the system.
Finally, you can add binary obfuscation / anticheat mechanisms used commonly in video games to ensure that even if someone is familiar with the application in question they cannot alter the certificates your application will accept.
Lots of e.g. mobile banking apps, etc. do this for maximal security guarantees.
Lots of apps use the anticheat/obfuscation mechanisms added by mobile apps are also trivial to bypass using instrumentation - ie frida codeshare. I know you aren’t implying that people should use client side controls to protect an app running on a device and an environment that they control, but in my experience even some technical folk will try and to do this
All our apps had to auto-disable pinning less than a year after the build date, because if the user hadn't updated the app by the time we had to renew all our certs... they'd be locked out.
Also dealt with the fallout from a lovely little internet-of-things device that baked cert pinning into the firmware, but after a year on store shelves the clock battery ran out, so they booted up in 1970 and decided the pinned certs wouldn't become valid for ~50 years :D
You will also increase the risk that your already understaffed ops-team messes up and creates even worse exposure or outages, while they are trying to figure out what ssl-keygen does.
Running certificate authority is one of those a minute to learn, lifetime to master scenarios.
You are often trading “people can sniff my network scenario” to a “compromise the CA someone setup 10 years ago that we don’t touch” scenario.
However DNS challenge allow for you to map an internal address to an IP number. The only real information that leaks is the subnet address of my LAN. And given the choice of that or unencrypted traffic I'll take that all day long.
Let's start with the obvious; wifi. If you're visiting a company and ask the receptionist for the wifi password you'll likely get it.
Next are eternity ports. Sitting waiting in a meeting room, plug your laptop into the ethernet port and you're in.
And of course it's not just hardware, any software running on any machine makes the LAN just as vulnerable.
Sure, you can design a LAN to be secure. You can make sure there's no way to get onto it. But the -developer- and -network maintainer- are 2 different guys, or more likely different departments. As a developer are you convinced the LAN will be as secure in 10 years as it is today? 5 years? 1 year after that new intern arrives and takes over maintainence 6 weeks in?
What starts out as "minimal private VPC" grows, changes, is fluid. Treating it as secure today is one thing. Trusting it to remain secure 10 years from now is another.
In 99.9% of cases your LAN traffic should be secure. This us the message -developers- need to hear. Don't rely on some other department to secure your system. Do it yourself.