In 1965 when the SM58 was introduced, the live audio industry was still very young and uncomplicated. One of the things I’ve always disliked about the SM58 is it’s advertised “frequency response tailored for vocals , with brightened midrange and bass rolloff”
Extra 2k to 4k might of been helpful if using the mic with a PA that didn’t have any HF drivers. For example, the Shure Vocal Master PA introduced in 1967. No high frequency drivers at all.