Firstly, we have the console manufacturers. Their consoles are first and foremost about playing PC games on console, so an imminent danger is that gamers interested in those games move to the PC about halfway through a generation, because the specs of consoles are locked whereas PC hardware is modular and can be upgraded whenever a gamer sees fit. Sony said as much themself:
So one fundamental idea of the mid-gen upgrades is to keep gamers in the ecosystem to have them keep buying games and paying for subscriptions; sales that would otherwise be lost. But a mid-gen upgrade can also be an incentive for new customers who didn't own the original model. Either way, it helps to prolong a generation, even if it's only for about one additional year that can be squeezed out.
Secondly, we have the big third party publishers. While it's more work for developers to have more console configurations, the publishers are fine with mid-gen upgrades because they prefer the closed ecosystems of consoles in comparison to the open PC. Whatever helps to get more money from gamers, hence why PC versions of their games are commonly not given the same care as the console versions, because there's more control on consoles.
The bottom line is that the PS4 Pro and Xbox One X weren't real success stories, but they weren't failures either. They sit inbetween as revisions that were worth making because they did at least a sufficient job for their respective manufacturer. As such, you can expect Sony and Microsoft to release mid-gen upgrades for the PS5 and XSX as well.