Apparently either he does not know how to serve properly or he just doesn't want to do it and wants to sell you a new string. I do believe however, a string could be replaced every year however, if you care for your string with routine waxing, it can and should last a couple years. You should be the judge of that.

When I purchase a new bow, especially the past monofilament serving, the first thing I'd do is remove the factory serving and reserve the string, it's easy. Plus, when you purchase man made strings like gibblet's or whomevers, it's not a factory string and sometimes better....reserving a string for years of use is not difficult, he just needs to want to do it.

Since you ordered the new string, ask him if he'd reserve the existing string and you can have the new one for immediate backup.