[jean_claude]

  > The idea of using special client to connect a server is somewhat alien to the generation that equates the Internet with the Web.

I think the problem wouldn't so much be needing a special client (app) to access a server (walled garden) as having to discover the existence of USENET, select a client, set up the client to point to their ISP's NNTP server (do ISPs even have NNTP servers these days?) or a premium or free 3rd party server, and then find an appropriate newsgroup(s) to follow.

Walled gardens also have the attraction of having a more user-friendly experience that can work across a variety of devices. The last time I was even moderately active on USENET, I was slurping newsgroups via a scheduled job running Souper and reading/writing posts on Yarn, a console mode program for OS/2. I'm fairly sure SLRN is console mode as well... not really a selling point on today's technology, where many people would be using their phone/tablet.