When I started wearing tefillin, few people used a small hand mirror to see whether or not it was properly centered. I recall men using the shiny metal area indicating where to push on a door, the window in a door to a darkened stairwell, and other awkward solutions. Compared to that, the current ubiquity of mirrors, whether in the tefillin bag or even glued to the bottom of the tefillin box is a G-d-send. But for most of Jewish history, mirrors were not cheap to come by. So what did the Ribbono shel olam expect us to do?
We lived for millenia before the heter iska allowed someone to give someone else money in a mechanism that allowed him to make money on the deal. The current interest free gema”ch is laudable, but we no longer feel the sense of brotherhood of “achikha ha’evyon” (your impoverished brother) that the Torah speaks of receiving your loan. Not to the extent that someone could buy a home off gema”ch money. Jewish society decayed, and workarounds had to be provided to minimize the impact of that decay.
Without the mirror, the only way to fulfill the mitzvah of tefillin correctly is through areivus, each person in the minyan taking responsibility for each other’s tefillin. Tefillin actually underscored the unity of the minyan, and the brotherhood of all Jews. But Jewish society decayed, and workarounds had to be provided to minimize the impact of that decay. The mirror is a better solution than trying to catch your reflection in a doorknob.
But now that we have mirrors, all we can see is ourselves.