Imagination and wishful thinking
The 150 AU mark is essentially meaningless – there’s nothing out there to mark the distance.
All those images on the internet explaining the probe's path indicate lines in space. Therefore, there must be some line out there. Are you sure that there is no 150 AU mark out there in the cold and dark? All those images indicate so many lines, which suggests that some kind of traffic control out in space may have been established(*).
But I guess we all can agree that the longevity of the probe is a homage to engineering and science(**).
(*) maybe it is for marking the intergalactic hyperspaceway?
(**) maybe the Vogons can appreciate engineering and science when they find the necessary paperwork in triplicate under the gold disc.