A hundred-dollar bill is lying on the ground. An economist walks past it. A friend asks: "Didn't you see the money there?" The economist replies: "I thought I saw something, but I must've imagined it. If there had been $100 on the ground, someone would've picked it up."
There were public order books years ago and there were still arb opportunities because the field was niche and people just hadn't written the software yet. In terms of your comment, the technical barrier was what "average investors cannot efficiently surmount." At this point the high-volume pairs are too crowded but I suspect that there are still opportunities to make money where technical complexity is still high (e.g. smart contracts).
There were public order books years ago and there were still arb opportunities because the field was niche and people just hadn't written the software yet. In terms of your comment, the technical barrier was what "average investors cannot efficiently surmount." At this point the high-volume pairs are too crowded but I suspect that there are still opportunities to make money where technical complexity is still high (e.g. smart contracts).