One of two things tends to happen.
1) The 20 is a fundamentally different problem, but is itself solved by applying different new developments (e.g. most breakthrough devices: semiconductor manufacture and design, cellular phones, iPhone)
2) The 80 is so useful that work realigns to avoid / ignore the 20 (e.g. most mass market adoptions: horse drawn carriages, trains, automobiles, planes, television, personal and business computing)
The key thing realistic futurists harp on is what drives revolutions is superior utility versus the status quo.
If AI is "same as human, without the labor costs," it will be adopted gradually. If AI is "better than human in a fundamental way," it will be adopted overnight.