That's urban myth.
Ja had two gold and one platinum album be released at the height of 50's career (which would be 2002/2003 - 2005) and one or two singles that charted.
The truth is even without 50's emergence the public was already starting to lose interest in Ja and Murder Inc.
50 blowing up like that didn't help but it was inevitable.
It happens to everyone with a few exceptions.
If you really look at hip hop you have 5 years at best before the public starts to get bored and look elsewhere.