mice aren't always killed when they die - Familiar HP over the decades

[Plane]

RUE 123
"Both the Shifter and the familiar gain an additional six Hit Points as a result of their mystic connection"
"If the animal is killed, the Shifter permanently loses 10 Hit Points (the six from the link and four additional)."

RUE 217
"Because of the magical nature of the union, the mage and the familiar both get an additional six Hit Points.
"If the familiar is killed, the sorcerer permanently loses 10 Hit Points."

Kill usually implies an unnatural death - if a mouse lives to the ripe old age of nine years old (most don't last more than a year) and dies of old age (natural causes) we usually don't describe them as "killed".

TTGD 48 for example suggest brown bat or vampire bat as familiar giving them an average lifespan of merely 1 to 4 years, while giving the black rat 2 to 4

So how do we deal with this situation? Why would a mage who plans to live until his 90s ever take a mouse as a familiar every ten years if he's guaranteeing he will permanently be out 40 hit points?

I'm thinking in this case (a natural death) we should at least ignore the four additional HP that is lost.

As for the initial six... interesting idea but ... what if we allowed a mage to keep it, if they managed to keep the familiar alive that long?

We'd see a lot more familars that way, because mages are wagering "sure I might lose 4 permanently, but I might gain 6 permanently" and aside from the extra spies, it would explain why greedy mages might start collecting armies of familiars because they want to maximize that permanent HP by getting as many as possible.

It would also explain why villains might procure big fortresses (to protect the precious bats and rats!)

There are situations of course where a familiar can't outlive the mage, TTGD pg 46 for example lets you sacrifice 5 PPE to give it 'long life' to prevent 'natural causes' deaths during a master's lifespan. The Familiar Link spell is oddly not referenced in this book at all so it's unclear whether it's a prereq for using these rules.

You'd probably not want this for the usual FL spell (unless you really loved the creature) but in the case where you're investing permanent PPE that's an investment for its own sake more important than any long-term HP gains.

Heart of Magic pg 47-50 revisits some of the TTGD rules too. It notably seems to shift "is killed" to "dies" on pg 48 though, but also adds a new feature "the animal will live for as long as the Wizard it is linked with" and ignores needing to pay 5 PPE permanently for this feature.

both books seem consistent that mods can only be done at outset and not later levels when you gain more PPE to use though:
"Additional modifications cannot be made at a later date: the familiar's powers are determined when it is first adopted by the magician"
"A Wizard may choose to permanently spend P.P.E. at the time of the initial linking"

I don't really know why that's the case, I like the idea of being able to reinforce a preexisting servant, otherwise there's incentive to abandon them and create a new familiar with more powers as you gain more PPE later.

Clearly the idea of familisr getting extra lifespan wasn't explicit or apparent before, i view that as a HOM change and so I'm exploring hte pre-HOM situations of bat familiars dying on an annual basis and what replacing them means.

Assuming I go through 20 bat familiars in 20 years that'd be an extra 120 hit points though so maybe it's a big overpowered to let them keep the extra six HP though? But I def wouldn't wan tthem being at -80 HP for suffering the extra 4.