But I am
putting things into perspective. I use pieces of software that are orders of magnitude more complex than GOG's installers, and yet they manage not to annoy me to this degree. It is precisely perspective - that this is, as you put it, a glorified self extractor - that makes what would normally just be an ugly implementation detail a huge problem relative to its purpose. It's one of those "you had one job" type of deals.
Here you are a huge leap of logic, your are equaling the "inconvenience" the new installer cause you to some sort of "lack of competence" on GoG part while those are totally unrelated.
Yes that are way in which they could have made the installer that would have allowed them to reach their goals while at the same time not preventing them from being extracted with InnoExtract, but the important part is that if they didn't do it it's not because they are not "competent" enough, they did it simply because they didn't care.
And why didn't they care ? well for a simple reason, because being able to extract the installer without executing never
ever was a feature to begin with, it was never supported by GoG, never advertised as a feature (or a core values), and the fact you were able to extract the installer was nothing more than a accident, an accident caused by the fact they used InnoSetup and that there was some external extractor available for it.
So when they had to modify the installer, they probably never worry a single seconds about peoples using InnoExtract, they considered what was their needs and what was the most efficient ways for them to do it, that's all, that's what "not supported" means, if has nothing to do with "competence" of lack of.
They could have hired the most talented software architects/developpers, spend months/years designing a bullet proof ultra compressed state of the art installer that would have been the most bug free and technologically perfect piece of software ever created... but in the end it could have been totally unextractable by any third party software and would have caused you even more "inconvenience" than having to run an script to extract it.