These ratings would change over time, so an A+ rating in 2017 would not be the same as an A+ rating in 2019. I think that is very confusing. Is a second hand A rated model better than a new D rated one? Who knows.
I find that perfectly fine. Why should an old model retain its A rating when in fact it consumes a lot of energy compared to a newer model? Just print the actual consumption on the sticker to make comparisons between older stickers and newer stickers possible.