The context is nothing of special: two girls are talking about a magician that in the past was famous and well renowned, but not anymore in the present days.
One of them tell to the other one:
Maybe is (from shikashi, because the previous part it's clear):
"But was only a matter of time before her magic lose interest among the people. (the audience).
"長く通用する" is clear (Adv+Suru Vrb), it's about "はずもなかった" that I have perplexities.
Thk U in advance.