I, for one, touched on source compatibility, but my central complaint is that /.../ complicates the mental model of the language while also being a manifestly bad syntax for regexes, since matching slashes is very common.
If the answer to that is to use “extended” literals either every time a slash is needed, or all the time, it’s hard to see how using /.../ some of the time will be an aesthetic or legibility advantage.
(Incidentally, citing the legacy of ed, which is mainly remembered for its outstandingly user-hostile syntax, in support of proposed Swift syntax is… difficult to describe politely.)