This goes back to the I don't want to work on these games anymore thing I've said like three times in the last two pages now. Yes they did intend for a ton of people to die. The Final Hours App says they see everything's thrown in a galactic dark age. The files for the Stargazer have notes that say its 10,000 years in the future and it sounds an awful lot to me from that scene like space travel isn't all that common anymore.
Yeah, it is a dark age, but that does not mean everyone died.
The "explosion" that is released from the Relays in the cutscene is the EXACT same as expelled by the citadel, the relays are just a way for the range to be extended.
The "explosion" is shown to have absolutely no harmful effects on organics in the very same scene, let alone going supernova which would destroy every cluster that has a Relay in it, destroying not just some but all of galactic civilization.
Yes it was established that a breach in a working Relay would cause it to go supernova, but this is not a breach, the Relays in the cutscenes expell all their energy, and go completely dark as if off before they break apart.
Bioware shouldnt have to spell that out, if you are intelligent enough to understand most of what goes on in the ME universe, then an implied effect like this should not need to be thoroughly explained. Earth was not destroyed, the Normandy was not destroyed, the planet that Normandy lands on was not destroyed.
Not having extreme collateral damage is even mentioned in the Codex entry for the Crucible.
The Pillars of Creation which are about 4ly in length a peice were completley destroyed by a supernova. If every relay in the games universe went supernova, almost every single planet and life form (read: all intellegent life in the galaxy) within radius of the relays would be stardust.
That is obviously not what happened.