Soonish
Explanation
The Joke
The comic promotes (or riffs on) the book Soonish: Ten Emerging Technologies That'll Improve and/or Ruin Everything, written by Zach and Kelly Weinersmith. It takes a technology that's "just around the corner" (fusion, space elevators, brain-computer interfaces) and shows that it's been "just around the corner" for decades.
The Humor
The running joke of perpetually-imminent technology is a staple of both SMBC and tech culture generally. Fusion power has been "20 years away" for 60 years. The humor comes from the tension between genuine scientific optimism (these technologies really could work) and the engineering reality (making them work is much harder than the physics suggests).
Context
Soonish (2017) is a popular science book by Zach and Kelly Weinersmith covering ten technologies that could transform civilization. The book is characteristic of both authors' approach: genuinely enthusiastic about science while clear-eyed about the obstacles and potential downsides. Kelly Weinersmith is a parasitologist at Rice University, and the couple's collaboration reflects SMBC's blend of scientific rigor and humor.