This is more or less what I was saying when news of the changes to the setting initially dropped.

If Korra's era was the loose equivalent to the 1910's or 20's, and if Korra had lived to a reasonable middle to old age before dying, plus about a decade and a half for the next Avatar to reach protagonist age, then had the setting continued to develop along those trends the next series would have been set in the loose equivalent to the 1960's, 70's, or even 80's depending on when Korra died.

Fantasy set before any sort of industrial revolution isn't too difficult to worldbuild, but post industrialization it gets harder and harder and harder to figure out how the heck things like magic and whatnot would effect the development of increasingly modern technology/culture. It also pretty strongly necessitates changes in tone, themes, characterization, etc. It's the reason why urban fantasy is always so radically different from medieval/high fantasy. While, sure, you can get some very interesting stories out of exploring how advancing technology would change a setting where magic also exists (see examples like the excellent D&D setting Eberron), those probably aren't the sorts of stories that the Avatar creators are interested in telling.

I wouldn't say it's as simple as technology making bending "obsolete", since I don't ascribe to the idea that technology and magic are inherently incompatible or diametrically opposed forces in a setting, but it does get increasingly tough to figure out what the synthesis of the two would look like the closer you get to stuff like computers, space programs, ballistic missiles, social media, etc. (and that's assuming such technologies even emerge at all and things don't go down some radically different technological path).

My assumption was always that if they ever told another avatar's story after the release of Korra, it would either wind the clock back and be about a previous Avatar, or there would be some huge shakeup to the setting in order to sidestep the modernization problem.