Where is the limit from which it is not worth adding more to an old car?

Using an old delivery vehicle in the company - a bus that is 15-20 years old, but which does not drive every day, only e.g. 2-3 times a week, usually longer routes 200-800 km, over the years more and more money is added to various repairs and maintenance it is in good condition, and at the same time the risk of breakdowns on the road and tow trucks increases. On the other hand, I know that typical transport companies (i.e. the car is on the road all the time, the engine does not cool down, only the drivers change) replace such buses with new ones after 3-5 years. Is there any objective measure that will tell you when to stop adding to the old car and replace it with a newer one? When is it cost-effective, because buying a new car is a big expense?
Using an old delivery vehicle in the company - a bus that is 15-20 years old, but which does not drive every day, only e.g. 2-3 times a week, usually longer routes 200-800 km, over the years more and more money is added to various repairs and maintenance it is in good condition, and at the same time the risk of breakdowns on the road and tow trucks increases. On the other hand, I know that typical transport companies (i.e. the car is on the road all the time, the engine does not cool down, only the drivers change) replace such buses with new ones after 3-5 years. Is there any objective measure that will tell you when to stop adding to the old car and replace it with a newer one? When is it cost-effective, because buying a new car is a big expense?
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