That's a cute grumpy old man take but I don't think it really holds up, not as a main cause of the desertion from Usenet at least. It's true that Usenet arose during a time when people using computers actually understood how they worked and how to use them, but there were also a lot fewer people on the internet. I won't hazard a guess as to in raw numbers whether the number of people who understand computers rose or decreased, but even if it decreased the fact is that there are tons of people today who were on Usenet in the day and no longer are, even though they presumably know enough about computers to get on it. Insofar as simplicity of access matters (and oh, how it matters) I suspect it's not just about how people back then knew how to do harder things, but also that everything was harder. The differential between getting on Usenet and getting elsewhere on the internet was less large than today, where the internet overall has gotten much more user-friendly and Usenet has not.
Offhand I'd guess a more salient factor is discoverability. In order to get onto a forum you need to 1) learn that it exists, 2) be interested in checking it out, 3) check it out and 4) participate. How do people even hear about Usenet these days, let alone hear something that makes them want to check it out? When I think of it, my path to Usenet was via the TalkOrigins.org website. Even then I bounced off of actually getting onto the newsgroup for what might have been years before I finally succeeded. And that was back when ISPs supported newsgroups! How many other "portals" to Usenet newsgroups are there - I don't mean a web interface, I mean any website that a random person surfing the web might run into that would 1) let them know this newsgroup exists and 2) make them want to check it out/participate badly enough that they'll go through the many, many steps required to do so ?
Discoverability is even an issue once you are on Usenet. Here I am, a person who I think has a little bit of experience with the thing, asking on Kbin for people's recommendations because I don't have a way within Usenet to know which newsgroups are active and which are good. You have to trawl through the list, subscribe and then you find out, and the list is much too large for a layperson to trawl through usefully. I'm working with the advantage of vaguely remembering which newsgroups I liked and were humming back when I was there; I don't know how a total newcomer would manage. Maybe there are actual websites and portals out there that help, dunno if anyone has recommendations.