They repeat at least 20 times in ME3 that conventional victory is impossible, so of course it is 
I'm being sarcastic, the Reapers would have been defeatable if only tha brain cells of every person in the ME3 universe did not commit suicide the day after ME2 ended. And i refer to it as the "ME3 universe" because, really, ME3 just needs to be categorized on its own apart from the other 2 (great) games
Each level of ME content (ME1 --> ME2 --> ME2 Bridging DLC) seemed to give more reasons on how the ME universe wouldn't quite be ready for the Reapers, even as it did get more ready than before. Call them excuses, and you may be right, but I wouldn't say that everyone's braincells died or anything like that.
Species were clearly preparing for invasion. They were just expecting things like a united Terminus or the Geth, not the Reapers.
Tech was clearly being advanced. It was just not with the rush of Reapers coming within years, but a threat that could come within decades or more.
The Reaper threat was being heard of, but mostly from Shepard (who was being appeased then blacklisted then shut out so he struck it out on his own) and in backchannels of those 'in the know' (Shadow Broker, STG, elements within governments even if Council hid out on the Citadel). Geth and Reaper tech was so intertwined in ME1 that Sovereign was just taken as the geth flagship meant to impose fear and power on organics. The evidence provided about Saren was something that people in denial about galaxy-killing robots could interpret to mean that he was only tricking those 'stupid Geth machines' about being their savior and taking power for himself, using a Reaper mythology that Shepard was playing into.
The companions from ME1 were a range:
-Kaidan/Ashley returned to Alliance and were promoted and seemed to be in contact with Hackett and Anderson like Shepard used to. Its arguable they didn't do enough (to our knowledge) but they did try to scope out the Reaper threat, and then be a help in terms of being on standby for the arrival of the Reapers.
-Liara was part of Shepard coming back to life and then had her vendetta, but before/after it, she clearly was engaging in things that would have manipulated the galaxy in ways that would have been more ready for the Reapers than otherwise. As Shadow Broker, her primary goal wasn't sustainment of a power base, but instead diverting resources to anti-Reaper measures. We see examples of this on her desk in ME3 and its unlikely she didn't do similar stuff pre-ME3. This is enough for us to understand that by ME3, many scattered people were getting ready for the Reapers (whether they knew it or not) even while governments were officially mum and most people were still not aware.
-Tali went back to the fleet and the Quarians are aware of the Reapers (even if they're some of the most on the sidelines of galactic affairs) and she's clearly studying the Collectors, Dark Energy, Geth/AI, so what else do you expect? That it didn't produce superawesomekillReapers results is sad, but part of the point that the Reapers aren't supposed to be easily picked apart and exploited by just one person (even ME3 was a galactic effort with Shepard just as the arguble foundation).
-Wrex went back to Tuchanka and wanted to give the Krogan the chance that Saren failed at. Aside from being a part of the ship and perhaps part of the squad that saved the Citadel and perhaps Council, he and many others don't have sway in galactic politics yet.
-Garrus had as much sway as someone like him could in the shorter term. He was written to be in a pessimistic stance since Shepard's death, and then after ME2 he did try to convince his government as much as possible of something with little evidence.
I could go on about ME2 squadmates but frankly most of them hold little sway with the large scale of their respective societies and I don't mind the stories of them orienting more towards settling old scores/debts/regrets/etc in ME2 and ME3 and between both games, than being some loud public figures screaming for change. I think we should keep in mind that this is the scale of a galactic society; its hard to remember when so damn much seems to center on Shepard, but even he is written to not be so influential that everyone believes him on everything. There is stability to the galaxy that so many focus on, and for good reason. If everyone tried to mobilize and freak out about the Reapers at once, before the Reapers even arrived, it could have very well had the galaxy LESS ready for them than what we got.
People were nearly as ready as they could be in an environment that wasn't going to just admit that everyonesgonnadierealsoon. Everyone exhibited things like the stages of grief, and in ME1-ME2 the biggest one was simply denial (ME3 had some too, but the least). It could have been more positive about the galaxy if it had stuff like the Alliance being fully out about the Reapers and the Council admitting to them (the ending ME1 scene with them is more and more awkward in retrospect), but I also have some sympathy with the direction Bioware chose.
One sort of thing I can remember that should have counted was stuff like video evidence of the meeting with Sovereign, or the fight against the Collectors, or even Object Rho (optionally). There should have been enough there for people to just know. In fact, entities like EDI and the Shadow Broker could have drip fed the galaxy information about it all. Instead, we have people preparing more slowly and quietly than they should have.