Spirit damage also affects Skeletons and Draugur

Considering how easy it is to get Silver compared to Bronze and Iron, this is not an issue to me.

You don't have to get the sword you got plenty of arsenal to go with each with there own taste, but if one's desire to get the sword, for him it will not gonna be a problem for them

It really depends on the world seed and player experience and choices.

IMO silver sword is overpriced since there's no real use for it in the game. Only enemy in the game where I'd use it over Frostner is Yagluth, but the only advantage is attack speed, and I think the secondary attack with Frostner is more effective DPS than the full attack combo with sword.

But the biggest issue is that more economical and cheaper swords exist in every other tier/biome as well. Silver sword had that anti-undead appeal while the only higher damage alternative was the pure-slash damage black metal sword, but that's not the case anymore with the mistwalker sword which has better damage type split as well as utility effect.

tl;dr it's probably not worth upgrading the silver sword at this point, it's only good for mounting on your wall.