Now, I am speculating here, but speculating based on experience - my own and also experiences my colleagues have shared with me. I suspect that Mothers' Day was chosen once upon a time as the "last day to sing" because the second Sunday in May seemed like a compromise between stopping choir at Easter and asking choir members to sing at the end of the school year and into the beginning of summer. I certainly have done this with my youngest children's choir, though this group now comes back each year for Ascension.
Which brings me to my question for the day - and the point I'd like to make. Yes, we are going to lose choir members after Easter. But this does not mean we should let the world drive the church's calendar. Over time, I have learned that if I schedule my adult choir through Trinity Sunday each year, more and more of them stay for the whole year. Yes, our school choirs have to be scheduled around the school year. So we do need to make adjustments. But even if half the singers depart, necessitating easier music, the visible and audible continuation of the choir through the Great 50 Days of Easter and at the great feasts of Ascension, Pentecost, and Trinity Sunday will teach and encourage your singers and, more importantly, your parish.
It's worth doing. Keep 'em singing.