To increase the lifespan of roofs the ventilation must exceed the minimum requirements of the building code. TV homes meet but do not exceed the code.
Why aren't more vents installed you may ask? Because no one cares, or put more kindly no one values it.
Some here will unite against this post. Think about it for a moment. Ridge vents are common here. Each linear foot of a ridge vent has 18 SQ inches of ventilation. It's common to have 40 -60 feet of ridge vent or just 7.5 SQ ft of ridge vent. Hardly enough to support all the soffit vents (the air flows up to the ridge vents). Powered vents offer limited help as they steal air from the nearby ridge vent and not from the soffits.
I added six SQ ft of vents I expect that I added at least two years to the roof life or until the homeowners insurance requires a new roof. The cost was 80.00 plus labor. I used
handeehandyman@gmail.com.