You're right; you can't use the warranty, because you've voided it, so it's good that you didn't care about it.
And it's a remanufactured device, not refurbished. Very significant difference.
The percentage of 4S users with a wi-fi issue (ANY wi-fi issue, not just possible hardware ones) is less than 0.5%. One out of two-hundred isn't common at all.
The fix you paid $30 is also a temporary solution. But of course, the guy selling you the service didn't say that, lol.
Apple stores replace any hardware-defect iOS device without argument, within the hardware warranty. Not just this issue. Just like if you have AppleCare Plus, they replace any damaged iOS device without an argument. And if you've never used the warranty, then how would you know how the process goes at an Apple store, lol?
What kind of warranty did your repair service have, and is it in writing?
EDIT: I reread what you wrote. You couldn't 'easily' use your warranty? Why? Where did you buy the iPhone, and where are you now? The free shipping option from Apple for warranty service wasn't 'easy' enough for you?