Do you think a Smartphone with older android versions are totally insecure and useless???????
I have 2 Smartphones currently running one on android Oreo and other on android Lollipop. Still they are running smoothly and support most of apps in play store. Now I have bought new smartphone only for new camera features.
For your information, any company whether it is Smartphone manufacturing or any other can only provide features and services as per price of the product.
You have given the answer by yourself, with time the app support for older version of Android is not provided....and secondly some major bugs/new features which are previously missing are provided only in major android updates by manufacturer