The idea I was going for was that the devastation of capital worlds like Tarsonis, the twist of episode 1 in canon, is what kicked off the status quo instead. Not all of the worlds, but sufficient to weaken the Confederacy enough for rogue states like the Dominion to form.
The planets don't need to be glassed
completely, because I want to have possibility to revisit them a la canon (e.g. Tarsonis' cities left ruins rather than being melted to nothing) without positing that
terrans have implausibly fast terraforming and population growth. Mass cloning and nano scale manufacturing shouldn't let them get out of jail free.
Mar Sara is fine though. Tassadar decided to team up with Raynor to fight off the zerg since evacuating the population isn't a long-term solution.