Why replace him? Don't need to. Show opens with Alan & Jake coming home from vacation, and Jake just stands there - frozen - with his mouth open staring at the door, while Alan gets the bags from the car. Why? Because two of the hottest, sexiest, most beautiful young ladies opened the door - the ladies that now own the house (for what reason? Pick one.)
(Actually, there was a third female who ran off with Alan's brother and is now living on a "paradise" island in the Paci Indian Ocean (or something like that, doesn't matter.)
These ladies just happen to look and act like someone's "goddesses". (Matter of fact, the term "goddesses" can be thrown in one time, but not too strongly.) Everything is subtile - very subtile - but you get the idea that the lifestyle of the ladies are just about the same as the "goddesses" in real life, and Alan kinda just gets caught up in it - gawking at what's happening - and the babes that are making it happen - with Jake looking on (sometimes having to give fatherly advice to his dad). (The ladies could even be bisexual, but no one will ever know.) I mean, after a couple of episodes, you can take this show in almost any direction, BUT always remember - the show becomes - ever so slightly - a subtile parody (that's right, a parody for those who get it) of Charlie Sheen's lifestyle, and the show will get the best promotion from Sheen himself trying to dis it.
So you've got the babes without the ego, fresh (and younger) life pumped into it, and more audience - ONLY IF THE WRITING IS CLEVERLY DONE. After three weeks of Charlie's "replacements", people will be saying, "Charlie who?"
Ever tried to write something when your mind is racing ten miles ahead of you? There is so much I could say about this, but I think you get the picture. The future can be bright for 2&1/2. As for Charlie's future... Dixie Chicks.