... my idea was to make it part of the core and to make it a switchable setting (default value: off/NULL)).

Without ignoring Micha's opinions (minimalism, simplicity and tidy code are probably the most important feature...), a switchable setting is great! The first thing I'd do is to turn the switch ON.

Opening links in a new tab saves a bit of bandwidth/expenses, and an extremely practical behavior for lots of users. (I'm not sure about Smartphone users... but Smartphones should be outlaw due to safety issues first and foremost).