I think you might be looking at this situation through rose-colored glasses. When I was a kid in the 2000s, I loved reading PopSci and I distinctly remember at least 30-40% of it being ads throughout most of the magazine. And then, the last 20+ pages were all ads.
Looking even further back, I've had to do some research on 90s vintage computers lately, and every edition of PCMag I've dug through has been laden with ads. It's even more packed with ads than I remember PopSci being 10-15 years later.
Maybe it wasn't mostly ads, but it wasn't far off.
What has changed is the annoyances ads cause. Online ads load slowly, screw with the layout while you're reading, and are overall much more disruptive to the reading experience than a print ad could ever be.