why do we write and print with two different alphabets simultaneously? we do not speak a capital A and a small a …

Legibility, in practice, amounts simply to what one is accustomed to. But this is not to say that because we have got used to something demonstrably less legible than something else would be if we could get used to it, we should make no effort to scrap the existing thing. This was done by the Florentines and Romans of the fifteenth century; it requires simply good sense in the originators & good will in the rest of us.