We were told that the Reapers' purpose (well, not so much purpose but preference, I guess) was what you wrote in bold. However, why would you think someone built them to do that? It would be pretty silly to build machines specifically to destroy you (which, ironically, is what the third game said the Reapers were). I was under the impression that, like the Geth, they were sentient machines that, at some point, turned against their masters and used them to become stronger. Unlike the Geth, however, they won, and have since been harvesting advanced races at regular intervals ever since.
That, to me, made perfect sense, and I think it made perfect sense to pretty much everyone who played the first two games as well. Even if the Reapers turned out to have some hidden agenda, I don't think anyone anticipated that it would be as terrible as the one BioWare ended up revealing. I think most assumed that it would have something to do with dark energy (or was it dark matter? I don't recall) if there was a twist. You know, because that was actually foreshadowed, unlike the star child and all of the crap he spewed.
And honestly, there didn't even need to be a twist. Robots want to wipe out advanced life, not so much because they're evil but because they can, and because that's what they've always done. Does it really need to be more complex than that? Would you rather have a finale where you spend ten minutes talking to a hologram kid who drones on about stuff you don't care about or would you rather spend an hour in epic fights with giant robots knowing that the fate of the galaxy rests in your hands? To me, that's a pretty simple choice. Mass Effect was, at its core, an action movie. Shepherd was the action hero who, depending on how you played, either did whatever it took to get the job done or always did the right thing. BioWare deprived us of the final, epic battle that every good action movie has, and also deprived us of any number of amazing moments utilizing the choice system (something the second game did very well in the finale, even if the boss fight kind of blew).
