Not sure where you got the 20% number from, but the better way to think of this is that each kind of disk technology hits a plateau at some point. Further investment in that will not bring improvements in density.
Moving to SMR is effectively moving from curve for our existing technology that is about to plateau onto a new curve. The delta between the current curve and the new one does start small, but over time will take us to a significant density improvement. The slide shown by one vendor showed that eventual difference being closer to 3-4 times the density but that was more of a hand wave I would guess.
The drive vendors shied away from specific numbers, but they all agreed that SMR was a new foundation that other technologies will build on (not something that will be replaced in time).
Reading a crystal ball is hard, but is does seem like a promising technology to invest in :)
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