WWE deciding to put Mysterio and Cara together in a tag team was the best decision possible for the circumstances. The only logical result is Sin Cara and Rey Mysterio's tag team leading to a WrestleMania match.
After seeing how he's transitioned in WWE, or lack thereof―there is only one money match for Sin Cara in WWE. That match is against Rey Mysterio. The only stage for the match is WrestleMania.
Rey Mysterio in my opinion has stayed around too long. Better to leave a day early on top then stay a day late and have people ready to see you go. Mysterio's legacy doesn't seem to be doing any favors remaining active, but he still is a big draw to the kids.
So the two need to fight at WrestleMania in April. It's currently September. Best move is to pair them together.
To the young demographic, the moves, masks, lights, costumes―the two look like they just jumped out of a comic book.
Sin Cara and Rey Mysterio as a tag team allows Mysterio to aid Cara. He can communicate with him before and during the match. Mysterio also provides his repertoire to deflect attention in the match away from botches by Sin Cara.
The two of them paired together can be used for as long as needed until the appropriate time comes to begin setting up for a WrestleMania match.
The other advantage with them as a tag team is if injury or suspension comes into play with either, which has occupied much of the last 15 months for both, then it has a less detrimental effect on storylines.
If Sin Cara is in a feud with Cody Rhodes, Cara gets injured/suspended, the feud is killed. If the team of Cara and Mysterio are feuding with another team, one gets injured/suspended, there are more backup booking options available.
A WrestleMania 29 match for the two should be the last match for both in WWE.
It will be a big draw with the fantasy aspect of what moves they could do with each other. It features two mega stars in Mexican wrestling history. It's the last match either one needs.
Sin Cara has a hard time working the WWE style. In WWE, and most wrestling in America, everything goes left. You move left, apply moves on the left side. Its all counter-clockwise.
Sin Cara works the lucha style that is more north to south versus WWE going west to east. Factor in the communication issues and his opponents not used to the timing of taking some of the maneuvers he applies―it ain't working.
Rey Mysterio isn't winning world titles and isn't going to win anymore. Injuries and suspensions have added up. He can continue collecting the payoff of being booked and royalties―at the expense of his image.
Have an entertaining run as a tag team, being the split to set up WrestleMania, wow the crowd, pick up your check and call it a WWE career.