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.
The most popular typefaces are the easiest to read; their popularity has made them disappear from conscious cognition. It becomes impossible to tell if they are easy to read because they are commonly used, or if they are commonly used because they are easy to read.