Oh hell no it isn't "once". I've been running Linux machines at home for various purposes for well over a decade and I'm constantly having to learn new shit. Especially because some distros will break things with updates and you're on your own to figure it out much of the time.
Linux is easier now than it's ever been but it's certainly not nearly as user friendly as Windows or Mac. It's also an intimidating topic to approach with an overwhelming amount of info that the typical the user never needs to think about otherwise (what's a distro , what DE should I use, how do I install software on my distro, etc). the aggressively unsympathetic community doesn't help either.
Some of that isn't the fault of Linux but the truth remains.