This. If I were a Python developer (of the language itself), I would be paying very close attention to how PHP has handled deprecation and breaking changes.
I understand why some people prefer that, and why it's the way most languages go (Java does the same thing, just with way less movement in general compared to PHP), but it means that the language gets worse over time. Eventually, if you don't do that breaking change, the language will get replaced by something that doesn't need all the cludge.
And also the jump from 5.4+ to 7 is probably smaller than the jump from 5.2 to 5.3.
Although I think the PHP project does need to support versions for longer, the adoption rate of 7 is going to be quite rapid due to the low barrier of doing it, and the massive performance and language gains.
Bad analogy, but the point is that Python still worked for most people so the upgrade process was slow.
I think Go is going to have issues similar to Python when they make the jump to 2.0. The devs are already on record saying that BC will break.