I found this to be interesting reading on this topic!
I think that the mystical part, if it even exists, is in how great the impact of a really good mastering job can be. Just like you say, it's different now, with less quality in comes the challenge of making higher quality come out. I predict that if master bus scoped mastering is not gone by now, I find it likely that it will slowly fade away. With the lower quality in also comes the need to have greater control of it and to balance things more transparently. I am both a recording, mixing and mastering engineer, I do it all and I'm always fascinated by how easy it is with modern mix content to completely destroy the conditions for what a mastering engineer can do with it in the traditional way. With access to clean stems (with fx processing in a separate stem), usually no problems, with access to a stereo track with a lot of distortion - a whole different beast.
It's still a bit difficult to generalize like this, I would say that the mastering job has been undergoing a shift in terms of what it contains and I don't find it to be something negative. Over this many years, people in the industry have been brave to challenge the traditional ideas about what the impact of mastering can and should be. I do find that because of all digital tools out there, the expectations have slightly shifted in that more is expected from both mixing and mastering, but that part is kind of just a distraction. I am of the opinion that having a great balance in quality of recording and mixing and mastering is what engineers should be aiming for. Then each process is contributing with its own important magic. But when the magic is gone in recording and mixing, it will certainly not exist in mastering either.
