Soccer has been calendar year based for a few years now.
An argument I've heard, but not sure I'd advocate for, is to just follow the school year/academic calendar, which would be along the lines of Sep 1 (or Aug 15 where schools start on the earlier side) and later cuts off to the next/younger level down. The reason for this approach, which potentially aligns better for traditionally fall-centric sports such as football and perhaps even hoops/wrestling that start in late fall, is that it keeps classmates/schoolmates together as potential teammates year to year. I can see this being compelling to some folks while not being a big thing for others.
The reason sports are going to birth year is because parents have been gaming the system for a long time holding kids back for athletic advantage. Seen lots of kids start Kindergarten late and then the parents held them back to repeat the grade so they are now 2 years older.
Then what happens is in rec sports kids who started on time get dominated in sports and lose interest and you get less kids playing. Last year when my kid was in 5th grade he was going against a 6th grader who was as big as us parents in rec 5/6 baseball. How do you think that went when this 8th grader was pitching or hitting?
And I lived the soccer birth year rule transition with my oldest. For the record, anytime this is done it screws the older kids the but I believe it's best to just make the transition and be done with it. My kid was a U7 when it was announced. Half her team was pushed to U9 the next year.
Now many of those kids were dominating the team because they were 6-10 months older than the rest. When we went to U8 tryouts, I saw kids flashing in tryouts because they were properly flighted against kids their own age. The only parents I saw pissed off about the change to birth year were the ones who started their kids later for athletic advantage. Because now their kids were all on the 2nd teams when competing against kids their own age.