I can definitely see your point here, as I am an avid sports fan and I always want my favorite players to go out on top rather than tainting their legacy with with some poor performances at the end of their career. So I've thought about this with DT as well, and here are some thoughts.
To me, there are a couple distinguishing differences between the athlete and musician as far as this goes.
First off, I think the downfall of the athlete is much more dramatic/sudden as they get older. While aging absolutely affects musicians (especially singers and drummers), I think it is much more impactful on an athlete who relies on their body, speed, strength and endurance - almost by definition of the word athlete.
Second, I think the legacy of an athlete can be more negatively impacted by poor performance in their final years, especially in individual sports such as boxing and MMA. This is because the fans of these types of sports are so infatuated with wins and losses. MMA fighter Georges St. Pierre just retired on top. He fought for years at 170 pounds, and decided to move up to 185 pounds to fight for a belt. He ended up winning, and decided to call it quits and retire on top after a couple years of indecision. Because of his amazing career and going out on top, many people consider him to be the pound for pound greatest.
On the other end of the spectrum is MMA fighter Anderson Silva. He was considered to be almost invincible when he was in his prime. 16 straight wins, many of them title defenses. But after his first loss in the UFC, he decided to take many more fights. He has won some and lost some, but he looks like a shell of his former self. A fighter who was once universally considered the Greatest of All Time, he is seldom called that anymore. Had he retired before his first loss, that air of mystery would have led many people to continue calling him the greatest.
With bands/artists, there isn't as much of an importance placed upon things like this since there are no "wins and losses". They will still always be cemented as arguably the definitive prog metal band, because albums such as I&W, SFAM, and SDoIT will always wash the bad taste out of the mouth of a poor modern album, should they release one.
Also, to echo the sentiments of your OP about them being on top - I totally agree! From what I have heard of the tour, James is on fire!!! I ordered tickets to see them in PA in a couple weeks, can't wait.