Without a mechanism like UBI, however, wealth will get trapped into increasingly concentrated funnels and effectively stagnate. This is bad not only for the people that get left behind, but also bad in general for the economy as a whole as an economy sustains itself on the constant flow of back and forth between buyers and sellers and consumers and suppliers.
As more and more people get replaced through automation, the available pool of money from consumers decreases. If it gets to a point where this becomes too low, the economy effectively comes to a standstill as the products and services companies can offer, even as they become increasingly cheaper to produce due to the productivity gains from automating away human labor costs, stop getting bought due to insufficient funds from consumers.
How can you justify this statement given the relatively low unemployment numbers pre-Covid?
This sounds like the same refrain heard at least since the early 1900's: Productivity and automation will set us free! Redistribute capital and income!
On the more pessimistic end, some people have always roiled against labor-saving technology.
In either case, both are predicting some future where humans are less necessary. Funny how this never seems to happen.
There will always be short term pain from displaced workers, but in the long term we will always have a need for humans to work. Technology is a multiplier and supports human effort.