I am no expert here so don't take my word for it but....

From the precept that a magnetic field is created by substances that are composed of polarized ions, I think the answer is that different substances polarize and remain polarized for varying times, some better (longer and more completely) than others,

As far as guitar PUs are concerned I don't think you have a lot to worry about, A vintage axe's PUs may sound week next to some newfangled super high out put mega-metal PU on account of the latter having stronger magnets with more copper wire wound round them. This does not mean the new fanged PUs will sound better simply because they have a hotter output.

As far as the decay of earths magnetic field is concerned, that is again due to the nature of substance crating the field, In this instance I think it is molten iron which is in motion. As motion and polarity are not really compatible the field is decaying.

Lordathestrings would be a good person to ask about this and probably knows considerably more than me !