You lost Mordin? Noooooo. Damn, seriously, like Freya said, he gets some great bits in ME: 3. I completely understand keeping it as it is though, just know that I would absolutely go back and re-do it.

The mechanics for survival is strange and points-based. It all boils down to holding the line. I'll just quote the wiki verbatim, as it does a good job of explaining it:

Quote Originally Posted by The Wikia
The teammates you do not choose are automatically assigned to hold the line against the Collectors. A 'defensive skill' value is calculated for the 'hold the line' group as a whole, based on a defensive value inherent to each character. Loyal characters gain a bonus to this value. If the 'defensive skill' check is not high enough to ensure all characters' survival, squad members will be designated to die based on their vulnerability, with Mordin taking priority if he is present.


Leaving behind strong defenders, such as Grunt, Garrus, and Zaeed will increase the group's odds of survival. In contrast, certain characters such as Jack, Tali, and especially Mordin have less skill in holding their position and should be removed through escort duty or taken along. The entire group will survive as long as the combined defensive skill is high enough, otherwise the game selects one or more squad members to die based on the scenario of who was left behind and how loyal the group members are.


Squad members who are not loyal are most likely to survive here than anywhere else, provided the group's collective defense skill is high enough, although they will be the first to die if the skill is not. Leaving behind two strong defenders such as Garrus, Grunt, or Zaeed should suffice, provided you haven't lost too many people.
The order of death is: Non-loyal, Mordin, Tali, Kasumi, Jack, Miranda, Jacob, Garrus, Samara, Legion, Thane, Zaeed, Grunt

I think what happened is you sent Thane (who, for a terminally ill drell) who seems to be the third most defensive character on escort duty. You also had Grunt come with you, which cut your defensive potential by quite a bit. Mordin was holding the line and not taken along, and thus died first.

None of it is your fault, you weren't to know whatsoever, which can make it annoying that someone died due to an algorithm. I usually bring Miranda and Garrus/Jacob/Kasumi, depending on how I'm feeling. I always send Mordin away on escort, though his incinerate is great against armour, so I might send someone else to do that next time.

Sucks that you lost Mordin, but good job on completing the game! I'm glad you really liked it!