Why wouldn't the "fat std" thing work? Yes it's hard to design properly, both in scope and actual design (especially for an unstandardized language still moving fast), but throwing the towel and punting the problem to the "free market" of uncurated public repos is even worse.
It's what we call in France "la fête du slip".
PS: that's one reason I try to use git submodules in my Common Lisp projects instead of QuickLisp, because I really see the size of my deptree this way.
Fat std library mistakes/warts would likely result in third party packages being used anyway.
Because fat std is rigid, impractical, and annoying.