Linux, Blender, and WordPress immediately spring to mind as software that would be in a very different place if their codebases reverted to public domain at the 10th year of their existence.
The Linux kernel has changed a lot in the last 10 years. Having all the code in it that's >= 10yo become public domain would only mean you'd be able to run an ancient kernel on old hardware without worrying about the GPL license terms.
How many are still running kermels from 10 years ago. even for mainline stuff with insignificant changes such that it is out of copyright (an interesting legal question itself), there is enough that is significant in new kernels
It's not about running a 10 year old Kernel, it's about a trillion dollar corporation owning a source snapshot, throwing 5,000 engineers at it, and not contributing anything back.
It also effectively turns GPL3 to GPL2 on a rolling 10 year basis.
People freaked about Tivo 20 years ago. Now imagine what kind of chaos Nvidia and Oracle could cause starting from even Ubuntu 14 or a 3.18 Kernel.
I'm surprised at people falling back into the BSD, MIT and GPL banter from 15 years ago.
Stop promoting your faves, stop generalizing about the motivations behind your non-faves, and to paraphrase John Lennon: imagine no licensing.
Now think a little deeper how that would change the motivations of developers, massive corporations, and VCs. Especially those that have given little but lip service to the whole movement.