From RSS reader developer perspective (I'm author of BazQux Reader https://bazqux.com) problem with RSS/Atom is that there are many feed generators (and even handwritten feeds) each with its own understanding of standard or without understanding at all. And good feed reader need to handle many quirky issues to support all kinds of feeds.
With new feed format there will still be old bugs and problems (duplicating IDs, missing IDs, new IDs for the same posts, duplicated posts, misconfigured web servers, servers down and so on). And new format needs to duplicated efforts like MediaRSS for extended information about podcasts (which JSON Feed is missing).
Problem with RSS is not a format (end users won't read XML/JSON anyway) but needs of average user, time to setup, vogue, no profit for big players and so on. People, not tech problem.
From RSS reader developer perspective (I'm author of BazQux Reader https://bazqux.com) problem with RSS/Atom is that there are many feed generators (and even handwritten feeds) each with its own understanding of standard or without understanding at all. And good feed reader need to handle many quirky issues to support all kinds of feeds.
With new feed format there will still be old bugs and problems (duplicating IDs, missing IDs, new IDs for the same posts, duplicated posts, misconfigured web servers, servers down and so on). And new format needs to duplicated efforts like MediaRSS for extended information about podcasts (which JSON Feed is missing).
Problem with RSS is not a format (end users won't read XML/JSON anyway) but needs of average user, time to setup, vogue, no profit for big players and so on. People, not tech problem.