I don’t think this will change anything statistically.
I have no numbers but as a French, I know literally nobody who have not been to school/kindergarten at 3.
Starting school at 3, while not mandatory before the law, was just the norm for barely everyone.
If the government mandates kindergarten attendance, well, now it's much more important to put funding for kindergartens in your next bill.
If every parent of a three-year-old has to think about finding a kindergarten, that's a lot more political clout behind getting better kindergartens.
And so on and so forth.