And the type of underperformance would have to be sub $500m per film worldwide. Otherwise there'd be no reason not to release them.
I imagine the issue is more about budget. The sequels are getting $1 billion, roughly $250m each. I wouldn't be surprised if over half that was spent on 2 & 3. So James is probably looking at a scenario where these films perform well, but not at the level of the first. If they make under a billion each (worldwide), then I imagine the studio is going to want to scale back the budget for 4 & 5. There's likely a provision in the contract they signed that has specific thresholds. And James Cameron is getting a cut of the revenues, but likely after the budget is recouped.
So I wonder if this is just a worry that the films won't be top 10 all time hits, and then James is looking at spending another few years making 4 & 5 on a tight budget, with execs breathing down his neck, and knowing he's not going to make a windfall from them. He may be setting the stage for a renegotiation. Or maybe he wants to move on to something else.
Could also depend on merch. I don't think Avatar 1 merch did well. That's a big, big driver for Star Wars, MCU, Pixar, and most Disney films. Even things like GoT were bringing in billions each year off of merch (though that market basically dried up after that finale, literally killed a multi-billion dollar franchise overnight). So if Avatar 2 & 3 perform well, but have no long term prospects for merch (at the level of other franchises), then it could end up being something that Disney sees as cannabilizing its other offerings. Once 2 & 3 are released, and Disney has their Avatar offerings in their parks, there's little that 4 & 5 can do to boost them.
Maybe 4&5 are best put on the back burner, until some "revolutionary" technology (glassless 3d) becomes available for theaters.