The fashion for using "authentic" pronunciations for originally foreign names is a recent one. Browning could and did rhyme 'Calais' with 'malice' quite comfortably, and Maria was Ma RYE a until the end of the 19th century, more or less. Peking has only recently become Beijing.
As for Saintjohn, aka Sinjin, think about saints and churches. In British English certainly, one would worship at Sin Thomas's, though not on Sin Valentine's Day. As a ln, and later a fn, the emphasis in Saintjohn is on the first syllable, leaving the second one unstressed; instead of the usual schwa (upside-down e neutral vowel), as we would expect for a middle-to-low vowel, the influence of the i in the stressed syllable raises the schwa to an i.
QED?