So you’re fine with ~/Downloads and ~/downloads coexisting as entirely separate directories? And John.McCauley@yahoo.fr and john.mccauley@yahoo.fr being attributed to two different people ;)
Second one: no, emails are not filenames, and more generally distinguishability is more important for identifiers. In cases where identifiers like emails need to be mapped to filenames, like caches, they should be normalized.
Case (in)sensitivity for filenames is a non-issue in my experience. Never had problems with either convention. As for emails, I do think insensitivity was the right choice.
The local-part of a mailbox MUST BE treated as case sensitive.
Section 2.4 RFC 2821, https://www.ietf.org/rfc/rfc2821.txt
do capital letters have a good enough usage case to justify their continued existence?