Argyle King
Legend
I feel like this is just hitting the difference between descriptive expressions versus proper nouns. The latter is meant to be descriptive, and can vary based on delivery and intent, so that your emotion will change the meaning within the group.
This is... less so with the former example. I'm reminded of the South Park episode where they tried to defend a more "neutral" usage of the word, without really addressing the fact the power it has as a general curse is derived from the homophobic power of the original. The slur is specific, and trying to generalize it is hard, especially when its power is derived from the original.
(Man, that episode has aged horribly in retrospect.)
And that's part of the problem: "phylactery" is a specific noun. There is a generalized form, but it has been almost completely superseded by the specific form. It's hard not to reference the specific, especially when you can see the (admittedly muddled) intent to make that reference. While it may not have been intended as offensive, I think it's hard to argue it wasn't used carelessly. This is why making up words (like the example brought up earlier of "Horcrux") is a safer and generally smarter option.
I think something like "horcrux" sounds cool. To me personally, "Soul Cage" sounds like a 90s alt-rock band.
I believe that how things sound can matter in regards to how well they catch on. "Bae" (to reference a significant other) hasn't had as much widespread appeal or staying power as similar words -partially because of how it sounded. While it still lingers, my perception is that it didn't have the same impact as things such as "boo," "boo-thang," "babe," "hubby/wifey," or similar words.
The reason I brought up the negative (f-slur) example is that it's an example of something being taken and used in a very negative manner (as phylactery being used in a negative manner). Yet, the perceived change in meaning to become a slur has been accepted as part of contemporary language. I see that as an interesting case study when contrasted against the idea that a change's acceptance depends upon being a positive shift.
In the upthread phrase example, a phrase comparing something to burning feces means something overwhelmingly positive.