Three's Company lasted another five years without Somers, so that is hardly jumping the shark. The same with Cheers it was the #1 show for several years with out Shelly Long, but in the latter case that was more because it was ensemble. Kelsey Grammer, Woody Harrelson, and Bebe Neuwirth picked up the slack after Long left. The reason Three's Company lasted was John Ritter he was really funny and the star of the show and Don Knotts was funny during those years. Somers was one the one who left that show and she acted like a major bitch when she let and her and the actress who plays Janet still don't speak.

Actually the season with Sommers fiasco saw a ratings slip, when she was finally gone the show had better ratings for the next two years and remained a top ten show. Don Knotts and Larry were awesome replacements, and I actually thought Janet got so much hotter by the end of the show.

The point is, Charlie was the foil and star of the show with the other characters playing straight to him. This is like the last season of Laverne and Shirley when Shirley just vanished. The show still remained in the top 40 after being a top 10 show and then they just cancelled it.

The point is no matter the replacement, they would never have higher ratings then if Sheen returned, first episode back could do 20+ in ratings

It centered on him, but people watched because of Ashton.

Another comparison is Happy Days where Ron Howard vanished. I do think the show is near the end we are looking at two more seasons at the most.