As far as I know (or as I was taught at school in France) it is Charlemagne who introduced bicameral usage in the latin script with more easily legible "lower case" (which were not actually "lower case" back then of course) letters for most of the script while keeping upper case letters at the beginning of sentences or important words also to improve legibility (serving as cues that help keeping track of where you are in the text). This was basically modern script, only the lower case fonts have evolved somewhat.

Capital letters in modern script are therefore not a remnant of technical limitations, the mix of lowercase/uppercase was actually voluntarily introduced for legibility once these technical limitations were gone.