I used to think that there was absolutely no way that CSI could go on without Petersen/Grissom. He isn't the reason I started watching the show, but he's the main reason I stayed, in the beginning.
But since then, there have been some excellent episodes that I have enjoyed greatly, and they have been no-Grissom, or Grissom-light. And wonder of wonders, I suddenly didn't care.
I think it would change the tone of the show in a big way, but I also think it could survive, whereas I used to think there was "No Way" that it could without him. I adore Griss, but I also think he is no longer the hub of the wheel in the show, as he once was. WP wants to leave, in his own words, and I think the show reflects that, now. I think it will be a vastly different show without him, but I also now think that it will still be entertaining enough that folks will still tune in. Never thought I'd see the day that I'd say that, but there ya have it. I know many of you aren't old enough to remember the original days of the show "MASH", but when a couple of very main characters left the show, alot of folks assumed it sounded the death knell of the series. Lo and behold, it didn't. Folks still tuned in, and the show went on to become one of the longest running TV series in history, and ended with a !Bang! by grabbing the highest rated TV finale in history to this day, I believe. (Of course, that was before the days of cable, so folks didn't have much of a choice as far as what to watch, so that drastically skews the numbers. But the fact that the series flourished after losing "Henry" and "Radar" makes my point). Yes, CSI will survive post-Grissom, should they choose to do so. But the question is, will they (TPTB) actually choose to do so? CBS certainly won't have the ratings juggernaut that they have on their hands right now, but they'll still be in the Top 20 with a Grissomless CSI (I think), and for that reason alone, CSI will still stand behind the series post-Grissom. But if it drops lower than that, they'll dump it, without a doubt, especially since they still have Miami/NY doing very, very well in the ratings, and are newer shows that show no signs of losing main characters yet.