I suspect you're right. Do agree they pose an unfair advantage, seeing as you have to be wealthy to go there?
I suppose it depends on how replicable those factors are for much less money. The signalling could certainly be replicated by a degree from a Russell Group university, and as you say, I doubt the subject matter knowledge is really that much different. The connections may be a factor, but that would only apply at the start of your career as it's perfectly feasible to develop excellent networks on your own, especially if that good degree opens doors for you. Likewise, good parenting would hopefully provide the confidence to strive for the best.
Of course, that isn't to say that someone attending Eton might not have those things easier than someone that doesn't, but I wouldn't say what they have is unobtainable by any means. Equally, I'm not suggesting that there isn't an old school tie thing going on in the upper echelons, but merely that not attending Eton should be no barrier to having a very happy life (whatever that means to you)