I’ve made the realization that this generation doesn’t understand that sales have predetermined end dates. I blame it on the fall of newspaper circulation. Sunday papers used to have all the weekly ads for all the retailers and it was implied that by the time you get the next ad the last week’s sales will have ended. Now people hardly even touch printed ads, don’t look at the fine print, and think sales should go forever.
It's been this way for some time now, right? I think you're right about this being in part connected to the decline of folks paying (any) attention to the Sunday papers, but that's a trend that's been going on well before even the last generation.
I tend to attribute the "endless" sales moreso to the sharp declines in sales of physical media. With digital storefronts now the dominant norm for games, there is no longer a need to coordinate with retailers on sales. Publishers can put things on sale whenever they want, and however many times they feel like it. There are no set end dates, and platforms seem to be fine with letting publishers even decide when/how much to discount titles.
All of this has IMHO contributed to the FOMO atmosphere for physical releases, whether they be games or merch. While there may be more sales and more choice, there is always this thinking that physical releases are less likely to come back and at reasonable prices. Digital products should never be scarce, though of course, Nintendo recently opened up THAT can of worms with its Mario anniversary game.