That overused proverb sucks because it's based on truth. Nothing lasts forever, and things are best-savoured when they're actually happening in the here and now. Yada, yada and indeed yada. It can be true all it likes, but that phrase hurts;nobody likes letting go of the good times, and no-one enjoys seeing great wrestling gimmicks die.
It's inevitable though. Looking back through the archives to pinpoint exact moments some of WWE's best characters stopped being as effective as they once were was a painful experience. The wrestlers here drum up so many positive memories from childhood or stand out as recent blessings in an industry that has, for the most part, tried to become 'less gimmicky' over the years.
WWE's ever-changing creative braintrusts aren't the only ones to blame for these nosedives. Sometimes, the performers themselves took missteps, either through boredom or because they believed it was time to try something new. Some recovered to have significant success after the initial brilliance of their characters faded.