To some extent, that was true about Unix for quite some time. I could argue that it's just a continuous of it. It's not completely believable of course because the desktop is geared for everyone. That said, the kind of things you get in extensions is radically different than trying to build all those into the core and then have preferences exposing them. Extensions can change behaviour completely and override those hated gui designers ideas and you can turn those on and off as you want them.
The problem with flexibility is that is that causes bloating and then you have people who just leave because it's just too heavy for them. There are always people who believe there is some magical place where you get all the options you want and your desktop will be light, efficient, and fluffy. It doesn't work that way. Every decision point is a lot more code to handle. More code, more change of human error etc etc. In the end, the same people who want the options will bitch on here about how bloated GNOME is and how it's taking 80Ms of precious memory out of the 6 gigs they have or it can't handle their 512M Pentium II computer.