Most shops are not going to know every minute detail of exactly when a specific part is going to fail. If one set of rails is failing, they are going to replace all.

If they don't replace all and there is a failure down the road, who shall foot the bill?

Will the customer say, "I just had the upper rails replaced XX miles ago and now the lower rails need replaced but that's OK because the shop tried to save me $ on the first repair."

Or will they say, " I just had the rails replaced, and now the rails need replaced again, rip off shop that should have done the job right in the first place. "

Based on my many decades in and around the auto repair business,
Ninety precent of customers will state the latter.

Here is an analogy from another type of business. A home owner has a central air unit with a failed compressor. The system is 15 years old and still operates in heat mode. A HVAC shop is going to recommend that the entire system be replaced due to aged components that may fail at any time. This is done to reduce the chances of " I just ( 2 years ago ) spent $800 getting my whole system replaced ( compressor only ) and it quit again ( no heat ) , now they want to charge me again to fix it. "

A commercial customer ( apartment building ) is going to be more apt to taking a chance on a compressor only repair as they realize that other parts may fail but in the long run ( many units ) they will come out ahead.