As long as the combination of Red Bull and Max are as far ahead as they currently are, Checo, or someone of his caliber, is perfectly fine for them. Max can virtually win the constructors championship alone, and his teammate cannot get close enough to challenge him. As soon as another team gets close enough, or separates themselves into second, so that Max cannot do it all on his own, whether that be Mercedes, Ferrari, Mclaren, or Aston, will be when they have an issue. And there will hopefully come a point where Max will need his teammate to help him out rather than just driving 10+ seconds up the road and the only time we see him on the coverage is when he crosses the finish line.
Currently, it's the perfect scenario for them. Their star driver, who has a history of being a bit petulant when challenged by a driver from another team, or his teammate, doesn't have to worry about being challenged, and they can still win the constructors comfortably.
Would love to see Alonso in the other car, he seems to have a healthy respect for Max but would not be afraid to get his elbows out and ruffle a few feathers. But it won't happen, Team Max would not react well to someone more competitive being put in the other Red Bull, we've seen how they (Jos) have reacted the few times Checo has dared to beat him, someone who could challenge him more consistently would cause them a real headache. Max would still win the drivers as he is the best out there at the moment, but the aggro and noise is something I am sure Horner would rather do without.
There always seem to be rumours about Lando and Red Bull, I hope this doesn't happen though as I like what Mclaren have going on and think Max would eat him alive.