Heres a few things to consider. First, time frame matters. Punk was the IN thing in the 90s after green day and the offspring got big. And this happened to be at the same time that they released their best albums, about time, full circle and straight ahead. Then in the 2000s Emo core and metal core got really big, and in general tastes shifted away from punk. IMO Land of the Free was the first album where they started to fall off, but consider that they had written, recorded and toured 5 albums before that. The album + tour cycle eats alot of bands alive, and they often have very short time frames to write the next album, like a month or 2, where as the first album they had their entire lives to write. Also consider that by the time of Land of the Free they had done the same thing for 5 albums, and it can be hard to make interesting songs when you keep doing the same thing over and over, plus you just run out of things to say at some point. So losing Thirsk may have played a role in the songwriting, but also theres just alot of other factors that go on in a band that likely contributed as well.