seriously, the glasspack is a straight through tube with louvers, behind the louvers in the body of the muffler is fiberglass matting that absorbs some of the sound vibrations. if anything this matting will either burn up or become saturated with carbon etc. from the exhaust. this will cause it to loose it's ability to absorb, therefore making the exhaust louder.
True with a car. Back in the day, with say a '57 chevy with a 327 / 350, glasspacks would need replacement every 2-3 years or so. Not so much with a Valkyrie when equipped with glasspacks that were made for a V8. They don't get so hot with the much smaller Valk engine, and last much longer. Feedback has been, virtually no one wears them out with the Valk. Though I have seen a couple comments about that - I wonder if that was really the case (measured sound with a decibal meter) or just a perception thing, like a butt dyno. Of course there's exceptions to everything - and very high mile bikes - 6-figure miles - are more unusual; we don't have a lot of results from that situation. I have sold only a couple sets of glasspacks as replacements, after having built well over a thousand systems in thirteen years - and one of those replacement sets was "just in case you get hit by a truck".
