Author here. This is definitely something some WISPs are worried about. It's likely that Starlink will take a lot of the most rural customers. WISPs can also be very successful in more suburban environments, and can speeds much higher than Starlink can provide in suburbia (300-500mbps) at a lower cost, so there will probably be a place for WISPs at least until there is no gap between fiber availability and Starlink as the best option.
Also, a lot of WISPs have started running fiber in rural areas, so that's another way they can stay relevant.
Also, a lot of WISPs have started running fiber in rural areas, so that's another way they can stay relevant.