Assuming that both the red and green phosphor are deteriorating at the same rate, the thing you have to remember is that if you increase the proportion of blue light, you also have to increase the proportion of green light to stay on the color temperature curve.
I am not exactly sure but I think it could very well be possible that even if you remove both red and green by the same percentage, the resulting light could still adopt a magenta color tint.
One additional minor contributing cause is that a portion of the blue (and bluish green) light in an LED comes from the phosphor, not the LED chip. This is only a small percentage of the total blue light (maybe 20%) but still could have a significant effect. This is especially true at lower color temperatures (2700-3000K). Once more of the blue light coming directly from the LED chip manages to get through the phosphor, it is going to result in a color shift in the blue light. A standard 450nm blue LED can appear very "purple"-tinted compared to the overall hue of the blue light in a white LED. Some LED streetlights may even be using 440-445nm blue chips (behind the white LED), which would make the situation even worse.