As I achieved platinium for 2018 in september (and as there are no roll over nights with Accor), I am enjoying Hilton Diamond and its free breakfast until the end of this year (I won't be hilton diamond next year). I must say It really makes a difference.
I believe the only reason why Accor doesn't offer breakfast for Platinium (in Europe at least) is the very large number of hotels compared to other brands in europe (for example in France 8 hilton hotels vs 1582 Accor hotels)