> IT/engineers won’t spend time building and maintaining email servers, we’ll spend $5/month/employee on G Suite and use their time on things that make our products more valuable

In-house email servers are not constrained by the time to build them, but rather by reputation/spam considerations. Sorry, but that's a lucky guess ;-)

> paying for a third party SSO solution for our products and instead using engineering time to build first class integrations between our products and complementary products, because having an artisanal SSO System doesn’t benefit our customers

That's one of vendorlockiest vendor lock out there, which is a very YOLO decision.

The idea that buying stuff is more efficient then building it yourself is at the cornerstone of the modern economic theory, but that's only a theory. In the wilds there's much, much more factors to consider then just comparing cost of building versus cost of buying.