Whilst I'm personally not overly keen on the changes, this really isn't the discussion at this point. What is done is done and we shouldn’t be flip flopping back and forth based on negative feedback after the case. Instead it has highlighted the need for systems both in terms of change communication and application features that can appease and benefit a whole bunch of different audiences.
I think this was a somewhat perfect example of the disconnect that can happen, as CalebJohn mentioned the amount of work that Laurent puts into this project is somewhat disproportionate and like many other changes proposed and discussed without comment, this one was also brought up to the community and went by without disagreement or any comment at all so therefore was justified. Obviously in this particular case it has become clear that the people affected were not of a demographic that had really been considered – as CalebJohn also mentions it was assumed that the main users would be either casual (who would likely welcome the change) or technical/enthusiast who wouldn’t think twice about tweaking or loading a css and therefore the change would barely affect them.
Posts like uxamanda’s theme discussion and andrejilderda’s macOS theme together with this situation shows that there is definitely a desire or even a requirement to make accessible user styling or theming tweaks that is available for all kinds of audiences. This would include a good, accessible starting theme to not dissuade newcomers, a good modular theme system that can allow less coding savvy users to still contribute to the project just as the plugin system has provided as well a system that allows the tinkering/enthusiast types to modify the application to suit their needs exactly.
I think overall that this is a good change even if for no other reason other than that it has shown just how much people do actually care about this aspect of the application.