Justify assuming first-order kinetics for a complex
What would be the justification for thinking this is first-order?


The KISS principle :)

Before you start to model complex kinetics, begin with a simple model. If it fits, consider your job done. 

Good enough is good enough!

(Imagine that you perform therapeutic drug monitoring. It's not your job to elucidate the exact metabolism of a drug here, you just want to be sure that the patients receive the right dose.)