With an open world game, the investment is much less risky
So I've got to disagree with you here. The investment would be more risky. When making an open world game, you need to make said world feel alive and everything that comes with that. Along with that the development times for open world games vs linear games is longer and they are generally more expensive to make.
I don't think that developers are switching to open world games because there is still a ton of games that are coming out that are not open world, a lot more actually. In fact if we're talking about big title releases I think the last open world game to come out ( aside from Saints Row) was Watch Dogs Legion. ( I maybe wrong on that.)
I'd like to see the actual figures but I think more people are interested in linear games, because not everyone has the time to drive from one side of the map to the other, especially when they have other commitments.