For how disasterous the Protheans' 'Plan A' (fight) was, the REST of their plans are why the galaxy survive.
In our cycle, we did all of the things that allowed this cycle to stand victorious! - Javik, probably
Attack the Reapers with the sheer might of the Prothean Empire? Nope, killed in about five minutes flat. But the rest of what the Protheans pulled off?
Begin studying The Citadel, the Relays, and Mass Effect technology. In no other cycle including ours did anyone question all this technology left behind by their predecessors enough to really look and see what makes it tick. The Protheans did. It's the decision that enables everything that comes after.
Build The Conduit. While this ran the risk of backfiring via Saren, having *one* mass effect relay that was completely outside the Reapers and Keepers control turned out to be vital. Since the prototype was one-way it also meant the Reapers couldn't use it or lock it down, when they arrived.
Uplift other species. The Protheans' greatest weakness was that everyone was a Prothean. Centralized record-keeping, an empire of worlds under one unified goverment, etc. But because the Protheans were interested in the species that became the Asari and Humans and Turians and Batarians and Krogan and even the Salarians (even if they DID eat bugs and their evolution was a surprise) et al et al - over a dozen - the Keepers would have to fight a war on multiple fronts, with disparate but united allies, exponentially raising the difficulty of the fight AND slowing it down.
The Crucible. Started by a different species multiple cycles earlier. Tested and refined by each subsequent one. The Protheans got the damned thing built but fell to infighting. They were also likely the ones to figure out the key was the Citadel - see above. They were able to finalize the design - meaning their successors could get straight to buildings - AND leave the record that factions and indoctrination had doomed the Protheans attempt.
Transmit a warning / use the Beacons. At the empire's height, the beacons had powered their entire galaxy-wide communications network. But now, those that remained served as equal parts message of safe harbor for any surviving Protheans and warning of what was to come if the message wasn't found until the next cycle. No beacon, no Shepard, and thus Saren and Sovereign eventually succeed at opening the Citadel relay - likely by Saren walking in through the front door.
Disable the Keepers. Another Ilos gem. The last actions of most of the last survivors of the Prothean Empire. The next cycle would never stand a chance if they got caught blind like they - and their predecessors - had. Cutting the connection between the Keepers and the Reapers bought the next cycle months - enough time to find out what was going on and prevent the blindsiding that had doomed cycles since time immemorial.
Stasis: On both Ilos and on Eden Prime, we see huge numbers of Protheans planned to go into suspension in an attempt to wait out the Reapers. The plan mostly fails, but delivers two key victories. On Ilos, the Vigil VI is able to save enough people to use the Conduit (above) to launch the mission to disable the Keepers (above). On Eden Prime, the Protheans are betrayed, but one is still able to be sealed away. Javik.
Who, though his own inclinations are usually the opposite of what should be done to maximize the war effort - his very existence is a victory. NO reaped species has ever, ever managed to survive to see the species of a new cycle. Even if the people of the new cycle are strange at best, finishing the work his people started 50,000 years ago is the ultimate in vengeance (which, after all, Javik is the avatar of)
Yeah, it's Shepard uniting the galaxy that wins the day - but everything Shepard does is built on the backs of the Protheans.
They're the embodiment of the old saw: "A society grows great when old men plant trees whose shade they know they shall never sit in.”
