Mark,

I think you may have misunderstood Keith's comment about 

it being "all a matter of time-frame."

He's right--when the sun consumes all the hydrogen in 

the hydrogen-to-helium fusion process and begins to 

expand into a red dwarf, that's it; there's no going 

backwards, no putting the genie back into the bottle,

no "renewing" the sun.  It's purely a one-way trip.

Now, as far as humans go, we're far more likely to be 

extinct due to other reasons before we come anywhere 

near to that point.

But as far as the physics goes, the conversion of biomatter 

into petrochemicals in the ground is more "renewable" than

the conversion of hydrogen into helium in the sun.

It's just that we're far more likely to hit the near-term 

shortage crunch of petrochemicals in the ground than 

we are the longer-term exhaustion of hydrogen in the

core of the sun.   ;)