The usage of RAR is totally minor aspect vs the issue that the archives are locked currently!
I would not downplay the RAR issue that much. At the moment we can extract the current
version of RAR archives. Being a propertary file format, we cannot assume it will always be this way. I would much prefer to rely on free formats for more futureproof archiving.
(BTW, the unrarlib library you pointed to was already a "victim" of a file format change. It only supports RAR format v2.0. Since September 2013, the latest version is v5.0.)
However, the main issue, as you say, is the use of deliberate
locking. We should, indeed, focus on that. For me, this is mainly a moral
subject, and I expected better from GOG.
I still trust that they will soon say something on this matter. But I already started looking for other DRM-free game stores and stopped buying games here. Just in case...