I think the series could EASILY survive the loss of Sheen. It is, after all, simply yet another variation of "The Odd Couple" anyway, and that basic premise will work with all sorts of variations.
Imagine: The retooled series begins with news that Charlie Harper has died. (Of something appropriate like alcohol poisoning, or being tossed from a bedroom window after being caught-in-the-act by a jealous husband) He then surprises everyone by unexpectedly leaving his entire estate to his brother Alan, whom we discover he didn't really mind having around (Cue audience to say "Awwww"!). Just prior to the mid-show break, Herb and Judith have a huge falling out (perhaps the result of some revelation made in Charlie's will).
The second half of the show begins as an obvious rip off of the opening montage of the old "Odd Couple" series, right down to the voice-over:
"On March 13, Herb Melnick was asked to remove himself from his place of residence. That request came from his wife Judith. Deep down, he knew she was right. With nowhere else to go, he appeared at the home of his friend, Alan Harper. Sometime earlier, Harper's wife, who'd later become Herb's wife, had also thrown him out, requesting that he never return. Can two divorced men, who'd actually been married to the same woman, though at different times, share an awesome beach house... and a son... without driving each other crazy?" (long pause) "Let the mistaken-for-gay-parents jokes commence."
Series repaired, and possibly improved.
