Context:
Permissive licenses (commonly referred to as "cuck licenses") like the MIT license allow others to modify your software and release it under an unfree license. Copyleft licenses (like the Gnu General Public License) mandate that all derivative works remain free.
Andrew Tanenbaum developed MINIX, a modular operating system kernel. Intel went ahead and used it to build Management Engine, arguably one of the most widespread and invasive pieces of malware in the world, without even as much as telling him. There's nothing Tanenbaum could do, since the MIT license allows this.
Erik Andersen is one of the developers of Busybox, a minimal implementation of that's suited for embedded systems. Many companies tried to steal his code and distribute it with their unfree products, but since it's protected under the GPL, Busybox developers were able to sue them and gain some money in the process.
Interestingly enough, Tanenbaum doesn't seem to mind what intel did. But there are some examples out there of people regretting releasing their work under a permissive license.
Temporarily free gives the idea that the code will stop being free at some point and may cause misunderstandings. It would be better to use nonreciprocal.
Because it absolutely can and most of the times does.
The original code never stops being free, but the code incorporated into a new project will be, so it's a misleading term to people unfamiliar with open source licensing, that may think the license somehow expires. Even the fsf doesn't use such terminology. They use reciprocal and nonreciprocal, because it translates the idea that gpl-like licenses create a relationship of reciprocity, and bsd-like ones create a relationship of non reciprocity.
It will stop being free the second a corporation gets it's hands on it, makes improvements and put's those under a proprietary license
Yes, but the originally free/libre licensed source code is still out there.
You could also make improvements and release them under a GPL license.