Let's not forget that Apple didn't release any public betas until Yosemite in 2014, the 11th release of MacOS X.
I can certainly understand a devs stance on not releasing betas to the public. I have served as beta tester for many software companies, and very few of them have public betas - you need to apply and agree to T&C and NDAs and so on.
And with a company such as Rogue Amoeba, they are usually ready on day one, so it doesn't really matter much. Even MOTU has gotten. better at releasing updates close to the official OS release date. I switched to Logic back in the day, because no one was sure MOTU would port DP to OS-X. This is before Apple bought E-Magic and there were rumors they may buy MOTU. It took quite a while as I remember for DP to run on OSX.
Of course, the OS is now more stable, and it isn't such a big change as it was going from OS9 to OSX.