I worked on a system implemented in Smalltalk for a few years, and the truly integrated development process possible in a ST is a high I've been chasing ever since. And this was in a setting that had so many things going against it: I started working on it in 2016, and the last update of the runtime environment was from '99. We had a custom HTTP client, and home-brew bindings to OpenSSL that I cooked up with some mad science Python that parsed C headers to generate Smalltalk code. I even had to fix a bug where Winsock fds rolled over if you opened more than 8k sockets, because the runtime FFI bindings assumed a 16 bit wide return value (I assume the code dates back to the Win 3.11 days).

By all rights, it should have been _awful_. But in reality we were able to iterate quickly and actually make stuff, because the technical foundations of the system were rock solid, the team was good, and because the truly integrated Smalltalk development enviroment enables a super-tight development cycle. A joke I have with a former colleague from those days is how all modern languages are still fundamentally no different from the punch card days: You take your sources off to the batch processing center, and the results come out at the other end, while Smalltalk is future techonology from the 80s.