I must be the only one who doesn't buy that "irrelevant" line.
When you have someone of his notorioty, you're bound to have lots of "listeners." Some of them are the diehards, who wake up at 5:30 AM, and follow the various listening threads. After the show, they go to MarksFriggin and re-read the whole thing. They'll do Google news searches about Howard and find even more. They'll go to the comedy shows, buy the various books, movies, and other promoted products.
This core group is who followed Howard to Sirius and was there on the first day. These guys probably even had Sirius long before 1/9/06 and listened to the various specials.
Your second group is the occasional fan. If Howard is on, they'll listen. If not, it's no big deal. If they miss something, they'll hope it gets repeated on Best Of.
These guys MIGHT have gone to Sirius for the first day, or they might have gone later. They may still be back at FM radio, or maybe they have iPods.
The third group is made up of the watchdogs. These people listen to the show with the sole intent of complaining to the FCC. If Howard mentions "his nether regions" itching, they'll complain since it's "corrupting the kids." If Howard says "passing wind" with those same exact words, they'll complain.
These guys don't care where he is now. Sirius? What's that?
I don't think Howard is "irrelevant" now, but he just trimmed the fat from the ratings. Asking your fanbase to pay for something which once was free is a good way to seperate the diehards from everyone else.
If there's a baseball team that's averaging 35,000 fans per game with $5 tickets, that looks pretty good, right? Now raise ticket prices to $35 a game and you'll probably lose some of those fans. Play the game in November with $35 tickets instead of June, and you'll probably lose more fans, seperating the diehards from the casuals. Would this baseball team be "irrelevant" because they are now averaging fewer fans per game?
In essence, Howard did exactly the same thing. You can't honestly believe that ALL of his ratings came from the diehards, can you? He took his product and asked the listeners to pay for it. Not all of them came along, and really, who expected them to?