What’s your definition of a “serious developer?” The vast European Smalltalk User Group (or ESUG) has thousands of members. None of them are “serious developers?”
Cincom and GemTalk are major Smalltalk vendors who’ve been in business for decades. Are they not selling their wares to serious developers?
Smalltalk has been used commercially for over three decades. Its major users include the likes of JPMorgan, Desjardins, UBS, Florida Power & Light, Texas Instruments, Telecom Argentina, Orient Overseas Container Lines, Siemens AG, and so on. Check out ALLSTOCKER and ATMs in Moscow streets. I guess none of these are serious developers.
In the early 2000s, the U.S. joint military used Smalltalk to write a million-line battle simulation program called JWARS. It actually outperformed a similar simulation called STORM written in C++ by the U.S. Air Force. Not a serious developer?
Smalltalk is currently being used to fight Ebola. This is serious shit.
The French Institute for Research in Computer Science and Automation (or Inria) are doing extensive development with Smalltalk. I think those guys are pretty damn serious.