[badlibrarian]

That effectively gives the copyright holder an exclusive ten year head start on a derivative work, because only they have the source to build on. It has to be open from day one for anything to work. And if you blow away copyright and therefore GPL, obviously the incentives change dramatically.

[Dylan16807]

I'm not really worried about someone building on GPL in secret while waiting for the license to expire. They're still stuck ten years behind mainline. It's a pretty even playing field, and I don't think any side gets blown out.

If proprietary code had to be released read-only a year or two in advance of becoming public domain you'd have basically the same effect, but I would not expect the effect to be very big.