My lesson from examples like this is that there's no point chasing compatibility. I was telling @Pezo - Zoltan Peto this the other day. I'm not familiar with Pypy, but I've seen several OSs chase POSIX-compatibility, starting with OpenBSD. Nobody cares. Inevitably they aren't 100% compatible on the axes people care about, and 99.9% compatibility yields 0.1% adoption. (OpenBSD usage is all by people who live exclusively in OpenBSD.) And people are right. They're being rational. They're following the design rule that similar things should look similar and different things should look different. If it looks similar but is different under the hood it's in an uncanny valley. It takes a lot to build trust in it. If you're building something new today, stop trying to support existing software. It's a long, thankless death-march, and will never get you the adoption you want. Take the leap and let it be its own unique thing, without constraining it with the past. New projects take 10 years (full time) to gain adoption. Stop trying to find shortcuts.

YES! I've been saying this for a long time: there are obviously use cases for OSes that do this, and POSIX exists for a reason, but POSIX-compliance shouldn't be considered the first step of building a new OS. If it eventually becomes something you NEED to have, build it as an extension, like Plan9 did. You might not be able to support the entire spec, but it will get you 90% of the way there which is good enough for most applications. Why should exploration of the OS space be constrained by "compatibility" dogma?

I remember in the early days of google, there was a big internal debate on whether they should standardize on Linux or BSD and internal support for each at the time was close to 50/50. I don’t recall what made them choose Linux, but I doubt it was POSIX compatibility.

@Kartik Agaram I agree for software, but any new system that can’t work with existing data and protocols ends up being useless.

Absolutely! https://futureofcoding.slack.com/archives/C5T9GPWFL/p1568821241254900?thread_ts=1568748321.207400 I'm a digital packrat; I wouldn't want to give up my data hoard.