Agreed. Also, it likely makes you and your company a bad Internet citizen to roll your own, for the reasons you've mentioned. My first company sold proxy servers, and I can't count the number of times a buggy router stack or other embedded thing broke the web for some users some of the time. If you make an off-by-one mistake in your PMTU discovery code, you're gonna waste somebody's day. If you don't deal with encapsulation right, you're going to waste someone's day. If you respond incorrectly to ICMP messages, you're going to waste someone's day. The list of things that can go wrong is endless.

I kinda feel like it's comparable to rolling your own encryption. Yes, most of the standards are well-written and well-defined, and you can spend a couple weeks really grokking Stevens' book (or whatever the modern equivalent is, I dunno, as I don't implement anything at that level anymore), but you're gonna spend years becoming bug-compatible with the rest of the Internet (or coming to realize your interpretation and the rest of the world's interpretation of the spec differ).

2 million requests a second sounds amazing. But, the price is high. As you note, if you have a team dedicated to it, cool. And, if you want to do it for fun and experience, that's also cool. But, Linux has a lot of wisdom built-in. There's like a gazillion commits over its life that have fixed various networking bugs and quirks.