Was Soichiro Yagami that much close to his natural lifespan? Shoichiro dies from wounds inflicted by Mello soon after making the Shinigami Eyes deal. Since he didn't even use the Death Note (nor was any DN used on the people involved in the incident) prior to his death, it can be assumed that the DN did not change the lifespans of anyone indirectly involved in some killing.

So because his lifespan was halved with the eyes deal and he dies a few hours/days later (lets assume he underwent surgery and a few days seems reasonable) was his natural lifespan that much short?
The answer is yes and no. He himself said that he was thinking about suicide and that he probably would have done it within a few days. At this point, his lifespan was that short. But his "natural livespan" is unknown (and was probably longer) since his son's use of the DN led him into this situation.