I *do* get the objection that Pascal had, and I absolutely agree that we should not choose a directory name that will collide with existing users' directories.
when reading this i imagined my future child some decades down the road reading the ASDF manual (hopefull only for historical reasons while reifying CL semantics to have an environment where he can try his father's code)... and then pondering: what on earth made someone chose this weird ~/asdf-local-projects/ name instead of e.g. ~/common-lisp/ :)
IOW, i'd like to draw more attentinon to the fact that what we are weighting here is a few active complaints today from experts (who are already aware of this issue reading the mailing list, even before the change itself)... against all the yet to be born/educated CL programmers until the time when ASDF is eventually obsoleted (or for that matter CL altogether), and all the aggregate time these future people will waste reading manuals and asking questions.
i think a tendency towards not taking this into consideration has a lot to do why the CL community is not successful. the defaults of slime is another very good example of this, although it got much better since the times when i first climbed the CL/Emacs fence.
also note: as Fare already pointed out, if e.g. XCVB gains more traction, then some/most systems will have two system definition files, but all sitting under the asdf specific ~/asdf-local-projects/ name.
but hopefully by the time my children are old enough for programming computing systems will have finally obsoleted this silly idea that the base axioms for data storage is labeled binary numbers, and their labels organized into a tree... and with that rendering this whole question moot. hopefully...
sorry for the sentiments.