Well, I guess it depends on the field you are working in. From e.g. a brain-surgeon I would expect to be as close to 100% mastery as possible. I wouldn’t want someone with ‘just 80%’ proficiency fumbling around with my cerebric, when he or she possibly could do better.
I agree that 80% is enough in a lot of cases, but some areas still require masters and will continue to do so in my opinion. Not every subject is evolving with such a rapid pace. And it’s not like you have to stick with the current level of knowledge for the next 20 years or so either. Things are changing, but you have to adapt to them anyway, whether they change today or in a few decades. You constantly improve on the most recent insights available(given they exist) to achieve true mastery, at least I think that’s how you do it.
Maybe the way to go for such cases is to stick with what you are doing and aim for 100%, while at the same time starting to gain some knowledge in other fields. I think it would be good for brain-surgeons to know a bit about psychology as well (don’t know if they not already have to in the first place). It would slow down your way to mastery, yes. But this is how you can become a jack of all trades, master of some, instead of master of none.