I am not going to say that some birds are not destroyed in the manner you describe, but I have stood on the loading docks of companies that are turning chickens into soup and dog food. So as for believeing you, I dont think so. Commercial chicken houses can house from 250,000 to 500,000 chickens at a time. All those chickens are the same age and stop laying at about the same time. Thats a whole lot of meat to turn into ash. Gassing and burying spent hens also isnt a economical use of spend birds, not to mention the amount of land needed to bury 1/2million birds a year, every year, for as long as the farmer is in business. A lot of those birds are rendered and turned into oil for animal feed, I suspect those are the birds you are being told are incinerated and buried, which is false. Another large portion of the birds are being cooked and ground into feed stock. 500,000 chickens make a lot of oil and feed stock, way to much to simply burn them into ash, and those birds are being replaced on a rotated schedule of around 80 weeks. Birds are replaced according to egg weight more than age since eggs are sold according to weight, and the goal of the farmer is to sell the most egg weight compared to the amount of food fed.
Definition of Spent Hen
A Spent Hen is a term used in the chicken and poultry industry to describe a laying hen that has reached the end of her productive egg-laying cycle, typically around 72 weeks of age. At this stage, the hen’s egg production decreases significantly, and she is considered to be of lower economic value for egg production.
Spent hens are often culled and processed into various poultry products or used for pet food and other byproducts, contributing to the sustainable and efficient use of resources in the poultry industry.
https://chickenpets.com/glossary/spent-hen-chickens-poultry