Lifespan is a factor in depreciation, not the other way around, although many people believe it is, i.e. depreciated equipment may still have usefulness though

Lifespan is a factor in depreciation, not the other way around, although many people believe it is, i.e. depreciated equipment may still have usefulness though some people (businesses) rush to replace equipment once it's depreciated. In case of real estate, depreciation offers income tax deferral because real estate retains much of its value and sometimes even appreciates despite being fully depreciated. It may be a part of or analogous to the borrow and die scheme of tax avoidance.