Edgar arguably won the 2nd fight against Henderson, but I doubt there are many anticipating a 3rd fight between the two of them. No matter how well Cain performs, not many are willing to pay for a 3rd fight since JDS would have already defeated him twice. He would just be widely regarded as the 2nd best fighter in the heavyweight division.
The difference there IMO is Edgar and Henderson fought twice back to back, no fights in between. People could be open to a 3rd fight between the two a couple years down the line. Just like it wouldn't have been out of the question to see yet another Maynard-Edgar matchup years down the line.
For Cain he just turned 30 and still has a few years left. If he keeps performing at a high level it's not out of the question he could fight JDS again down the line, just not anytime soon. Only scenario I see otherwise is if JDS gets hurt and there is an Interim Title made. I'm sure Cain would be Mir 2.0 and get the call for any Interim Title fight.