Why is anything important? One of the big mysteries in ME1 was who the Protheans were and what happened to them. It was assumed that they were destroyed, but in fact they were changed. It was an answer to a mystery.
Well, the Protheans weren't Collectors, any more than husks are humans. And Prothean civilization certainly was destroyed- though that was established back in ME1.
I'm pretty sure we didn't get the reveal of Reapers as a "species-box" until after we had met the collectors. Vigil on Illos had no idea of their motivations and no idea where they came from. My memory might be faulty on this, it has been a while.
But the reveal that Reapers are made of people doesn't really require the Collectors. If, say, Geth were processing human prisoners, the reveal would be just as valid.
The "arbitrary handwave" showed that not all species were suitable. Given that the Protheans, Turians, Asari, Quarians, Salarians etc were not, gives even more gravitas to how long they have been around and how many species they must have wiped out.
Except ME3 walked back even that. ME2's 'you are not suitable' got cast aside for ME3's 'only one dreadnaught, but lots of lesser Reapers from other species.'
The fact that not all species get made into Reapers doesn't really add any more gravitas than some of the planet scans or Leviathan of Dis in my view... but that, to, doesn't require the Collectors to demonstrate.
After all, Heretic Geth- who really, really want to be Real God-machines, won't be able to become Reapers. Yet another role the Geth could fulfill, even as any other genetic unworthiness could be communicated by the Reaper proxy.
Also it is relatively easy to control a bunch of robots. Their thinking is controlled by code, and however complicated is relatively, extremely easy to replicate and adjust. With an organic species it is much, much more difficult, and the Collectors did show much more intelligence than the human husks we had seen.
The Collectors didn't show any more intelligence than the Geth we saw in ME1, and far less than most of the VI/AI characters we dealt with. The only Collector that showed any intelligence was the Collector General, and that was because he was a Reaper puppet 99% of the time.
So could the story have been written with Geth instead of collectors? Sure. But it would have been a weaker story. Having a new species gave a level of mystery and the unknown to the second game. If it had been Geth again we would already know pretty much everything about them.
What did we actually know about Geth after ME1? The closest they had to a major reveal after showing up at all was that they had vague religious motives.