For me it's rather simple. I like GNU/Linux, the whole free/sharing ecosystem helped me learn so much, it allowed me great productivity, to modify anything and everything, etc.
I like others to have the same opportunity, so GPL it is, even for libraries.
Well it doesn't always work that way. I have a bunch of MIT licensed libraries that are very widely used and I'm 100% certain there are no commercial forks of it of any particular value. Because then they'd not have my support when their stuff breaks. Im sure there's lots of other libraries like mine, the MIT license is not limiting anyone's access to the best versions of my code.
Your argument doesn't say anything about GPL not always working that way. It just says that MIT license may work that way too, if noone else will make any useful closed modifications. Which is actually an uninteresting case.
I like others to have the same opportunity, so GPL it is, even for libraries.