Actually, this was probably a recurring thing that has only happened during the Mighty Morphin'/Zordon era of Power Rangers. It's just that since Saban at that time didn't want to change the cast until halfway through the second half of Turbo, the producers at that time felt that the fans weren't ready for a new start, nor were they actually ready for some new heroes afterwards. However, since then. Nowadays, the fans have actually accepted the newer cast members of different seasons and they really don't care who gets to be which ever Power Ranger as long as the formula stays the same, but also adding in some new mythos to the franchise. Besides, even if we do want to see some of the original cast members from Mighty Morphin' back in the newer seasons of Power Rangers, it's best that they only make a brief cameo appearance or just act as a supporting character/mentor character to the new team of rangers. Because, quite frankly....it's NOT their show. The old MMPR crew have had their fun, but it's time to give the new kids a shot. Let the newbies have their time to shine on spotlight. This isn't like what happened with Dino Thunder where Tommy has to work with some new kids aren't the same as the original team. No, no, no, no, no!!! This is something different.
So yeah, I wouldn't seeing any of the original Mighty Morphin' characters make a return to the franchise, but, let them act as a supporting character or just a brief cameo. That's it.