Many comments here are arguing that quality has actually gone up over the past decades. However, a common experience for me is that I own something of good quality from 5/10/15 years ago and now buy the successor model from the same brand, but the product has gotten worse, being cheaper made. And I have a hard time finding a replacement that matches the quality of the old version. It’s a regularly reoccurring frustra…
I'm sure that is certainly part of it, especially when multiple players are competing at least partly on price or apparent value. One consideration once price based competition is significant is that absolute quality may drop while measures of quality/price value improve¹.
Another issue is that when something is new, to the company or the buying audience, it is often a flagship product/service so gets a lot more attention. As things become something the company rattles off as a matter of course and we consumers interact with them daily, that level of attention per production unit diminishes considerably.
As well as playing directly into this, possibly leading to an actual drop in quality, mass production has a less obvious effect on the perception of quality. If you are making hundreds or less and a couple fail, they are probably noticed before leaving the factory and if not the consumer gets a relatively personal service with fixing/replacing the item. If you are making hundreds of thousands many more bad units get into circulation (the absolute failure rate increasing even if the failure ratio drops) and processing returns is less logistically easy. That perception problem has become more significant in the last couple of decades as unhappy voices always tended to be louder and social media can act as a megaphone for both happy and unhappy voices.
This is a complex area with many things feeding into actual quality issues, the perception of them, and sometimes the perception of the matter being worse than it really is overall.
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[1] for instance, maybe in a made up example quality goes down by 5% when cuts or mass production bring the price down by more than 10%, so buyers get better for the same money but worse absolutely