My guess as to why the Shibs decided to come back:
It would be hard not to notice that the level of dance is not as high as it was when the Shibs were competing. They were up against V/M & D/W at the beginning of their career and then P/C entered the mix. They won three world medals and two olympic medals, none of them gold. I imagine they may have seen C/B winning worlds the last three years and thought, hey we were consistently beating them when we competed and they are older than us. Perhaps they thought it was possible for them to win Olympics or Worlds or maybe even an Olympic Team Gold with the absence of Russia.
Idk, just a guess.