>If coding is easy, was Carmack just at the right place at the right time?

Yes, it's fairly axiomic that he was "just at the right place at the right time" because there are dozens of modern "boomer shooters" on steam that are not nearly as successful as DOOM or Quake. You might counterargue that its always harder to do something thats never been done before but that would only be a tacit admission that coding actually was the easy part in that case.