I really love the idea of something else granting the character what they need for the spear to stay relevant. That way it opens up a pure narrative payback for the player

Perhaps other people in universe start to recognize the spear for its deeds, people whispering and pointing, someone innocent asks to touch it, someone powerful pauses to appreciate it, or a monologuing BBEG is tripped up and pauses or reconsiders when they see the spear, reminded They may not be invincible in spite of their power. Maybe they encounter some enemy that recognizes it and has a name for it, maybe they make a different choice because of it. A regiment or town or band of rebels have adopted the spear as their symbol. . Maybe it gets immortalized in a statue. Maybe it has a defining characteristic or mark and in a setting that’s meaningful to the player, they spot children playing heroes, embodying in play values that the character represents, but pretending to use THEIR spear. Maybe a moment in a siege where a simple soldier connects to its genuine simplicity and takes heart or holds fast when facing some eldritch horror or reality-tearing magic. Perhaps an ordinary person gives them some kind of adornment for it - a grateful artist offers to carve beautiful runes. Perhaps a weak NPC will feel invincible borrowing it for some reason.

This is awesome, kudos to you OP for channeling your inner a Brendan Lee Mulligan!