Davening times

Question

I understand your point about the need to daven maariv after darkness if you are davening minchah late. This is indeed what one must conclude from the Shulhan Arukh, And yet, the minhag in Europe for hundreds of years was to daven maariv when it was still daylight. The poskim made their peace with this (see the first teshuvah in Terumat ha-Deshen). Certainly a custom that was universal in Europe for hundreds of years, and was tolerated, albeit begrudgingly, by all the gedolim, qualifies as a minhag vatikin, doesn't it?