This doesn't make a heck of a lot of sense. ME1 had plenty of missions that had nothing whatsoever to do with Saren. Your description only works if we don't play the side missions or if they don't count because reasons.
ME 1 had a coherent narrative. Like a Serialized show that continues the story from last weeks episode.
Side quests has NOTHING to do with a main story, it's fillers.
ME1 you progressed in the story in a linear fashion in the sense that you move from planet to planet to hunt him down, chasing him and the story evolves with new twists etc. Most of the missions you undertook was about Saren and where he had been and what he was up to.
Mass Effect 2 had a very VERY small 'main story', in fact out of 50 hours game time 2-3 hours was the 'main story'. The rest was just episodic content, such as; go fetch companion, then do companion mission that had NOTHING to do with anything. Just a filler to drag out a game with MINIMAL story. Set pieces to have shooting and bang bang action in it.
Just look at it this way, if you watch CSI TV show ( same for any cop show) THAT is episodic storytelling, each "mission" is self contained story. Some of these types of shows have a little 'main narrative' in them too, like X-files had their on going stories about a bigger threat (the alien stuff and mulders search for his sister). But most episodes were just self contained EPISODIC content. ME2 was exactly like that, separate companion stories in that you go and get them and then do a little mission for them. Interspersed with 1-2 main missions. It was pathetic.
ME1 all missions was main story, except the smaller side mission fillers.
Now go watch Game of Thrones, that is a narrative serialized show.
I prefer the narrative/serialized/linear type of story. That doesn't mean the world can be "open world" and also contain side missions/quests.
I really hope DAI is not like that. However that seems to be the Bioware motif as of late. ME2 and ME3 just being a rehash of the "story technique" .
Am I the only one that saw that ME2 and ME3 was basically the same thing? The gather companions in ME2, change it out to 'faction's in ME3, all in the name of fighting this "suicide mission' at the end.
So cheap.
This whole war table and the seemingly same story of the : "big threat, must gather the warden faction, mage faction, templar faction and your resource meter increases so your 'inquisition becomes strong enough to fight the main threat." PLEASE do not be like that, and IF it is like that, please have more main missions and deeper story.
Talk about recycling.