I'm not sure this is true, though I think it looks true.
I think the issue is that when a lot of people have put work into something you think that the chances of success yourself are low. This is a pretty reasonable belief too. With the current publish or perish paradigm I think this discourages a lot of people from even attempting. You have evidence that the problem is hard and even if solvable, probably is timely, so why risk your entire career? There are other interesting things that are less risky. In fact, I'd argue that this environment in of itself results in far less risk being taken. (There are other issues too and I laid out some in another comment) But I think this would look identical to what we're seeing.
I think the issue is that when a lot of people have put work into something you think that the chances of success yourself are low. This is a pretty reasonable belief too. With the current publish or perish paradigm I think this discourages a lot of people from even attempting. You have evidence that the problem is hard and even if solvable, probably is timely, so why risk your entire career? There are other interesting things that are less risky. In fact, I'd argue that this environment in of itself results in far less risk being taken. (There are other issues too and I laid out some in another comment) But I think this would look identical to what we're seeing.