While David Ruffin is more or less irreplaceable, the group found a way to replace him by entirely changing their sound.

With Kendricks, however, the group chose to hire second and third rate mimics rather than find someone who also had a unique falsetto (like a Russell Thompkins Jr. or old boy from The Delfonics). They didn't replace Ruffin with a mimic, so why would they think replacing Kendricks with mimics (no offense to Damon Harris and Glenn Leonard, but let's call a spade a spade) would work?

Even despite that, the reasons the Tempts didn't last long without Kendricks are several:

1. Losing Kendricks' behind-the-scenes efforts (the vocal arrangements and costume design) was almost as important as losing him as a lead. So was losing Paul Williams as choreographer and the "spirit" of the group

2. Without a decent and consistent relationship with a great songwriting team (because Barrett Strong walked out the door right after Kendricks and Williams), the group was resigned to second rate material. Meanwhile, you have the mighty mighty O'Jays over at Philadelphia International turning out monster hits.

3. By the mid-1970s, the Temptations were relics. They were in their mid-30s and had been on the Top 40 charts more or less consistency for a decade. Falling off was only natural at that point for a group like theirs.