this had been edited three times.
I am a mikimoto person too. i did not realize when i bought them that they have a finite lifetime. now I do.
in my opinion the sales associate did nothing wrong. with daily wear you wore out and damaged your mikis.
she was right to pull out a fresh strand. she gave you good advice to salvage the remaining pearls, and buy a fresh strand.
an analogy: like eating an apple, with core in hand, going to the store to demand a fresh one to replace it.
i also wear my mikis daily, but wash mine when sweaty to avoid the discoloration. needless to say they have been restrung because the string has gotten wet. when they wear out (five years?) I plan to try the hanadama quality freshwater pearls.