In season 2 (Since Will was mostly absent being lost in the Upside Down) Schnapp gave one hell of a performance as the scarred and then possessed Will. In season 3 Will has the (unfortunately cut short) arc of being left behind by his friends who are marching towards and embracing the changes of their teen years. He has scenes where he gets into it with Mike or where he destroys Castle Byers. What have they EVER done with Lucas anywhere near as dramatically rich or as interesting as that? The honest answer is that they haven't. Of all the kids he has the most subdued personality, whether by design or default, they just haven't done anything particularly meaty or interesting with Lucas. Thus, he finds himself mostly just kinda there. Sure... They give him an action to do, use his wrist rocket, come up with the plan to use the fireworks... But is that an arc? Are those textured traits of characterization? I would say no.