Each year new products & models are launching, so that those in need of it can aquire them. These companies are delivering OS updates for these smartphones so they last longer as realistically possible.
If when yours is broken or far too old, then you should consider aquiring this year’s model. So that you can use something that is compatible with studies, work, activities etc.
Obviously each individual/ family/ organization does their own analysis regarding if there is a need or desire to aquire said products. Also what for.