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.
Another difference is that the Edgar fights were competitive and exciting. If Cain gets finished again by JDS...it would be two violent finishes.