I do believe that Leander Paes is the best mixed doubles player we have right now. Due to his ageing body, he may not be as good in men's doubles but he plays mixed doubles absolutely brilliantly as can be seen in the recent results.
However, as you say if they don't have any chance of qualification, then there is no point of a debate and if Sania is going to play with Bopanna for the Olympics, she has to partner him at the French Open and Wimbledon. There is no winning partnership (unlike Paes who is going strong with Martina Hingis) that they need to break for this.
However, in the recent past, I read an interview of Paes where he was very optimistic about playing mixed doubles at the Olympics. His ranking at that point was still not good enough so why make such statements? Is there any other way he thinks he can partner Sania in the mixed doubles?