Regardless to what he is saying, I think he is experiencing moisture dampening his bow. In the winter the poundage will go back up. Not all the way, but it will go back up.
I have handles an osage bow close to 100 years old. It was the darkest reddest brown you can imagine. I bent it a few times without a string ( it didnt have one ) and it seemed fine to me. it was a straight and crudely made stick with deep side nocks and a rounded end. I recall it being a bendy handle and not very long. I was very tempted to offer to make a string to string it up. But, I didnt want to be the cause a 100 year old bow broke.
Another guy I spoke to told me he owns an 80 year old osage made by his grandfather. He wanted to know if it could still shoot. without seeing it I couldnt tell him. He was supposed to show me but never did.
If the bow is well cared for, and the wood is quality, I cant see them not lasting longer than we our selves do. Trees were built to flex. I dont think they wear out. Look at the 1000+ year old redwoods, and Joshua trees. Just past the sapwood they are all dead wood, as the layer just under the bark is the only living part. And they bend all the time in wind. I dont think a well made and cared for bow of any wood can ever die.
So now I must ask, what bow have you got you are worried about? Did you find some old neat relic?