I agree with you, all other things equal, a newer loudspeaker can be expected to perform better than an old one. People don't generally promote a product that is inferior to an existing model, at least successful people don't. So one can generally expect newer technologies to be superior to older ones. And where speakers are concerned, I have found this to be true. It is an incremental thing, no "leaps and bounds" improvements. And there are some truly excellent products from 20, 30, 40 years ago and more. But all in all, take a JBL speaker from 40 years ago and compare it with the current product offered for the same market today, and improvements are evident.But there are a couple of advantages that are still obvious for you.
1. Cost
2. Compatibility
You can expect the repair to cost less than a replacement, particularly if surrounds are the only problem. And since the cabinet and crossover were designed for this woofer, you will get best performance staying in a certain "window" of parameters near to that of the original part. If you simply repair the existing woofer, you will know that its characteristics will be appropriate. But if you choose to replace the woofer, you'll want to find those that are compatible.