It seems to me the incentive is mostly that it costs tons of effort to maintain these things in-house, and that it doesn't actually hurt the business to share (

It seems to me the incentive is mostly that it costs tons of effort to maintain these things in-house, and that it doesn't actually hurt the business to share (most) things. Does copyleft help? Perhaps, but that's not clear to me. There are tons of successful non-copyleft projects so copyleft certainly doesn't seem to be a requirement, and look at how many companies are violating the GPL and basically just keep doing it. While GPL suits do happen (just last week in France), in general they're exceedingly rare, and the risk consists of a bit of negative publicity among a small number of people. All of this is of course an old discussion, and to be fair I don't think anyone can be sure of anything here which is why people have been discussing this for 30 years. Basically we'd have to construct an alternative universe to be sure.