I think there's another angle to this, as discussed on the
ATP podcast, that it's just good for marketing.
But the purpose of marketing is to sell things. And it seems that the Thunderbolt display was not selling, even though it offered real value to MacBook Air (and to a lesser extent MacBook Pro) customers as it was also a hub in addition to a display. Of course, that value lessened as the product aged and it's ports went out of date, but then because it sold so poorly, Apple had so many they could not get rid of them (to make way for an updated model with modern ports) unless they "fire-saled" them and Apple doesn't do that because it dilutes the brand (marketing).
So Apple releasing their own 5K display would just do the same - people would complain how expensive it was and how poor a value it was (since the USB-C ports are not max-speed and it doesn't support Thunderbolt 3) and how people are idiots to buy it when they could get a "Dell and a dock" for so much less. And in a couple of years when USB-C is twice as fast as it is now and Thunderbolt 4 is out, everyone would bitch how Apple is charging so much for an outdated display.
The only marketing Apple gets out of being in the Display business (and router business, for that matter) is bad marketing.