My point is that I imagine a network designer shouldn't focus on Wireshark or tcpdump integration over other non-functional requirements such as, well, network performance.

Network performance isn't as visible as the non-functional requirement of inspectability because it is amortised over potentially millions of machines, whereas inspectability is an immediately visible issue to the select few who "pop the hood" to fix an issue or simply to have a look.

For example: in terms of network capacity, I wonder how much HTTP headers cost all of us collectively. Probably a lot more than the cost of making a Wireshark plugin and having sysadmins install it as necessary.

Edit: put another way, I think designers should prioritise the needs of the people who pay the cost of network operation over the convenience of the operators.

There's a feedback loop here -- if it's too hard and thus very expensive to operate a system, then optimising for performance was a false win. But I don't think this is such a case, especially since as you pointed out elsewhere there are a number of very mature binary wire formats that were extant in 2007.