let's not lose sight of the fundamental premise here.
One of the biggest issues with open, is that it makes it far easier to be an aggressor than to work together with a friendly player. If all you want is a hollow Square to shoot at, it's very easy to do that. Frankly, it's so easy to do so, that it would almost make sense to remove the Hollow Square icon entirely.
But the chances of meeting Another player who's doing the same thing as you are vanishingly small. So, naturally, even if you are theoretically interested in playing cooperatively with other players , you still wouldn't be incentivized to play in open, because it's probably not going to happen.
Naturally, there are some players who simply aren't interested in player interaction whatsoever, and this would have no impact on them, but there is also, I imagine, a substantial population of players who would theoretically be interested in Cooperative gameplay, but simply lack the means to acquire it. Giving them those means could draw a substantial amount of players into open, with virtually no downsides other than, of course, developer time. But any change would take that, so if you have to choose, why not choose the option that has the fewest potential downsides to the players?