[Skarg]

Seems to me ghosts are rare and only happen when the GM says they do, generally "some emotion so strong that they simply refused to leave this life", which to me seems very different from a player who is looking for a rule loophole.

The word "sometimes" in the line you quoted is in italics and to me reads like it's yet another indication it would be exceptional rather than routine. It also goes on to say, "The person thus returned to life will have a one-track mind until his/her ghostly desire is satisfied." which to me strongly implies that having a passionate ghostly desire is probably a prerequisite.

It also requires a greater Wish, which can revive you anyway. And it doesn't say anything about helping with aging.

This seems to me nothing like a technique "to live forever" (not to mention that it's a strategy that involves dying, and that something that kills you might kill your goblin friend too).

To me, it seems like if someone likes the idea of someone devising a way to live forever, it makes more sense to invent something with a flavor you like and add it to a campaign, rather than looking for a loophole in the wording of rules. Maybe I misunderstand you though, and this is a tongue-in-cheek way of pointing out a potential mechanic that might get GM'd weirdly and wants some different mechanics?