The server demand doesn't depend on how many servers you have. If your demand is D, you have X servers, and you need to decommission Y due to age, you'll have to buy D - X + Y. If Y gets smaller, you buy fewer.
Data centres fill up. They fill up with important "production" software and associated data housed in the same DC for locality, which never moves and mustn't be shut down. After a while the data centre is full and everyone needs to put their new software in the newly built empty data centre.
Over time a data centre gets technology upgrades later or not at all (say, a new-new networking fabric is invented and starts rolling out before the new networking fabric finishes rolling out). There's a slow decay in terms of service. New data centres have on-site offices of engineers who keep everything running at maximum efficiency, older data centres have a single security guard who receives a daily printout of which machines to press the reset button on while doing their rounds. Eventually an old data centre is turned off all at once.
If they're keeping older servers around longer, they may be delaying turning off the least valuable computers, continuing planning upgrades to 'new' even after 'new-new' is ready and rolling out, or slowing down the invention of new tech. As I understand it, "decommission X many machines due to age" isn't really the model.