Every roofer will tell you a replacement is safer. It is also more profitable, so it is worth knowing how the decision is actually made.
Is the field failing, or a detail?
Granule loss across the whole roof is age. A stain under one valley is a detail, and details are repairs.
How many layers are up there?
Two layers means a tear-off regardless, which changes the arithmetic considerably.
How long are you staying?
A five-year plan and a thirty-year plan are different decisions on the same roof.