I think down at a certain income, there's lots of people with an older simpler car that do at least some of their own work, or have a family member, or the handy neighbor work on it. The $100 oil change, doesn't make much sense when that's your take home pay for 7-8 hrs.
I think the trend of big "dumb" box stores with poor inventory management(walmart, etc) is that keeping track of the larger number of automotive basics, is "too hard" or too expensive?
I'm glad Canadian Tire here, got serious with their website and inventory system, so its searchable by store and updates pretty quickly, with even their auto parts searchable by vehicle. Makes walmart just a PITA to deal with, compared to that.
Probably more new car buyers are now trading in before major work is required, and never lift the hood, so they aren't shying away from complex vehicles, as they aren't keeping them for that long.