Sounds like you are new to all this, because they did only private betas for many years before they started public betas with Sierra. With a few exceptions such as 10.0 and 10.5, releases such as Jaguar, Panther, Tiger, Tiger for Intel, Lion, Mountain Lion, Mavericks were all private. Yet, Big Sur which was public has more glaring bugs that I am sure public testers didn't pick up. Its really no benefit to be honest.
While I agree that overall software quality and QA on Apple's part has gone downhill, with some egregious regression bugs lost in a seemingly eternal limbo of not being addressed for years on end on account of not being “critical” enough for Apple engineers to care, you seem to be failing to appreciate just how much bigger and more complex Apple's market is these days.
Also, FYI, don't let my badge and post count fool you; in my case, look at join date instead. My first Mac was an iMac G4 USB 2.0 which came preinstalled with Jaguar and bundled with Panther upgrade discs. Those were, indeed, “the days”, but we paid through the nose for upgrades (I bought a boxed copy of Tiger for €120, IIRC, and paid for all ESD upgrades on the MAS right until the last one, having only skipped SL because my 2009 iMac came bundled with it). And the same should go for everyone else here, even if it's the other way around or anything in between; maybe our fellow forum-goer simply joined us later on or even bought a Mac much, much later, but that doesn't mean they didn't reach the same conclusion just by staying informed – maybe even as far back as way before buying their first Mac, yes – and looking objectively at the current state of affairs.
The switch to Intel brought about an explosion of choice and different possibilities (remember Steve Jobs' old, cutesy 2x2 product matrix for the then recently revamped Mac line? Yeah, neither do I), as well as third-party software and peripherals galore. How many really critical bugs were related to the latter, with external hard drives getting mysteriously erased, audio interfaces suddenly not working at all, etc., remember those? Yeah, that's why public betas are essential, as developers alone can't cover the entire gamut of configuration combinations.