Those living during the decline of Rome were likely unaware that anything was happening. The decline took over a couple of hundred years. Anyone living during the decline only saw a small part of what was happening and likely never noticed it as anything other than ordinary.
Countries don't have genetically determined life spans. Nor do they die quickly, unless the cataclysm of some great war does them in. Even in such extreme cases, there are usually warning signs, which are more obvious in hindsight than at the time.
Few citizens of a dying nation recognize the signs. Most are too busy trying to live their lives, sometimes not an easy task. If death occupies their mind, it is with respect to themselves, a relative or a friend. Most cannot conceive of the death of a nation.A Country Dies Slowly First
For those interested, signs or symptoms precede death for a country often as they do for a person. There is a pattern that involves the following: