This is all opinion, but for me, it’s not that modern macOS has severely regressed from Snow Leopard, though I don’t like notarization and I also don’t like the flat design and iOS influences that have made its way to macOS in the past decade. However, it seems to me that macOS hasn’t improved substantially since Snow Leopard, at least from my point of view. This isn’t necessarily a bad thing; even with the iOS UI elements that have made its way to macOS, somebody from 2009 using Snow Leopard would easily navigate a 2024 Mac running Sonoma; the macOS interface hasn’t changed substantially since 2001. Other than integration with the rest of Apple’s ecosystem, most of the macOS features I use were present all the way back in Tiger, such as Spotlight.
Another issue I have with modern Apple software is the removal of features (I still miss iPhoto; Photos.app is inferior) and the loss of the perfectionism that once defined Apple; I feel Apple’s software has gotten buggier. Of course the Mac today is much more stable than it was in the classic era (no memory protection and cooperative multitasking made for an unstable environment that meant needing to reboot in the middle of getting work done), but it seems like a regression from the 2000s when Steve Jobs had tight control.
There’s also the context of when Tiger, Leopard, and Snow Leopard were released. I believe these releases of macOS were far better than contemporary Windows releases (XP, Vista, 7) and compared to KDE and GNOME of the time. In fact, if somebody made a Linux desktop environment today that was a clone of Tiger and if enough software was written that conforms to Apple’s human interface guidelines of the era, this would be heads and shoulders better than current versions of GNOME and KDE, but this is my opinion and everyone has different tastes.