PurePareidolia said:
The only difference between where we are at the end of Arrival and if Sovereign had succeeded in ME1 is the reapers don't have the citadel, and Shepard is a wanted criminal instead of a celebrated hero. We're effectively in a worse position than if we had, at the end of ME1, wiped the citadel census data, then let Sovereign summon the reapers as he planned.
If Sovereign had succeeded in ME1, the entire galaxy would be hit at the speed of a jump gate, with no warning, no preparation... no chance against the Reapers. Shepard would be a hero, yes, but even he can't do much against simultaneous overwhelming attacks on every inhabited planet in the galaxy right after the closest thing to a centralized government was eaten.
At the end of Arrival you know what's coming, where, when, and theoretically have the capability to fight it. No, the Council won't believe you and will probably work against you every step of the way. But if you did it right, you don't need the council. You have the Shadow Broker broadcasting your warning to every sentient in the galaxy whether the idiots at the top like it or not. You have the Rachni at your beck and call, a force that nearly wiped out all intelligent life when they were working alone and acting as mindless drones instead of an intelligent race. You're practically brothers with the head of the growing unified Krogan nation, the race that put down the Rachni and almost took their place as the galaxy's doom (and you have a scientist capable of redacting the Genophage if you need to). There's a good chance you'll be able to pull the full combined power of the Migrant Fleet and the Geth Hive in behind you if you play your cards right. If Sovereign had won, the Reapers would be facing a scattering of confused defenseless apes. Now, they're facing a galaxy that's armed, waiting, and almost unified except for a conspicuous hole in the middle.
That said, Arrival wasn't nearly as well put together as the rest of the game, had several stupid points (why a solo mission? Why all the plotlined stupidity?), and was seriously overpriced for the amount of content it provided. But the story is still good.