I think you have a point, although it is not quite that bad.
A good agent can help with the description, adding in stuff that is likely to tempt in buyers. A good photographer will partially stage the house and ensure it is seen in its best light given the time available. I have been shown round by agents who have really sold houses well, pointing out good things and explaining how stuff can be fixed easily (if it can). I have been shown round also by overbearing ones who won't leave you alone, and school leaver types who add nothing to the sale other than opening the door and surfing the net on their smartphone. There is also the after offer service in moving the whole process along, ensuring you get the sale and they make their commission.
So I think an agent can make a difference, although the difference is probably more limited at the offer stage for the reasons you suggest, everyone is on Rightmove.
I think the idea that agents have some sort of pool of buyers that only they have access to and when you take them on they will become available to buy your house is a load of rubbish, for two reasons. Firstly, all buyers are looking at Rightmove. Second if they do have some people who are somehow tied in with them they are likely to be investors who will only pay bottom dollar on a particular house anyway.
Still I agree with the general points that most houses are on Rightmove, that houses are in short supply and there is a vast pool of people want them (but who aren't necessarily able to pay for them), and if something comes on at an attractive price it will find a buyer. Similarly if something comes on significantly overpriced there is relatively little magic a different agent is going to be able to conjure up to increase viewings/offers and make it sell.