I agree that Paine is far from the #1 wicketkeeper in Australia.
However, CA and the national selectors are showing loyalty to him and have given him an extended run in the side solely due to the way he's held the team together since he took over the captaincy from Steve Smith. Purely as a PR exercise, CA chose the person they knew would appeal to parents with young children after Sandpapergate.
Paine would either need to reinjure the finger Dirk Nannes shattered a decade ago, or have an absolute shocker of a series for the selectors to force their hand in his retirement. Otherwise, they will let him bow out on his own terms, which most likely will be after next summer's Ashes series (he'll be 37 by then)