And he's probably not wrong. The amount of people who build PC's now is probably a tenth of what it was in 2000
Probably a tenth of the percentage it was in 2000, or an even smaller percentage.
And that's "PCs", then you add in tablets etc and it becomes an even smaller percentage.
There are millions of people in the world who have never even considered the idea of building their own computer because they grew up either with laptops, or with tablets. The worl has changed and moved on, and 99+% of people want a box that works.
Even enthusiasts here would/do buy things like Roku or similar TV boxes to plug in and have work, and buy laptops etc, which are just sealed computers.
Also OEMs can more easily make small computers than end users, and when people want smaller computers, that pretty much requires a sealed box (we're talking Mac Mini/Dell Zino/NUC size things, although NUC is quite DIY, but the size is what I'm talking about).
Sealed boxes are the mainstream, have been since forever, and DIY makes up a smaller proportion than ever before.