You mean Gibbon? His premise (that I disagree with) was that Christianity was the issue: Romans got too un-warlike.

I'd agree that the issue was mercenaries, but it wasn't that Rome couldn't attract average citizens, it was more that Rome got too big for those average citizens who joined to get home in time to plant and harvest - so their farms would fall into disrepair and eventually be lost to them - in spite of the victories those soldiers were winning each brought more territory into the Empire. Average citizens weren't able to get any of this. Each of the two Graccus brother tried to fix the issue - and was murdered for their pains. So the legions became primarily professional, and once they started hiring 'barbarians' those barbarians learned the 'secret sauce' to a Roman Legion's success, so the Legions lost their technological edge, and it was a matter of time...