Since we want to talk about mid-tier authors, I think you'd be surprised how long and how well royalty can still pay authors. Cassandra Khaw has repeatedly commented on how Hammers On Bone (published 2016) is still making them significant revenue. And this isn't at all covering the nuances of UK/US markets, where books can make it big in the UK and only years later publish in the US and vice versa (where I know for certain make significant income for authors)! I'm sure Susanna Clarke, who has Chronic Fatigue Syndrome, is still immensely grateful for how well Jonathan Strange & Mr Norrell continued to sell (and then she came out with another banger, Piranesi!). Victor LaVelle is still making money off The Ballad of Black Tom (2016) to the point of producing a re-release hardcover of the book due to popular demand! And I know Nnedi Okorafor is still earning excellent royalties from Who Fears Death (2011) even though her more profitable works (Binti, Akata series) are getting more attention nowadays.
The argument that books primarily sell 95% of all they'll ever sell in the first 2-3 years is a total misunderstanding of the book market. It may look like that for current-events nonfiction or celebrity bibliographical works, but fiction markets don't work like that at all-- the backlist makes a huge chunk of the stable income of an author. This is true also for indie and self-pub markets (I can't name specific examples because I haven't done heavy research into this, but I know from panels and talks from other romance authors that self-publishing romance heavily relies on voracious readers who will buy your entire backlist.)
But should any of those authors become sick, disabled, or otherwise no longer able to produce, should they end up never making another cent of any of their works? How would they support themselves 5, 10, 15, 20 years?