Bench players are bench players because they're usually either streaky as all hell or consistent in their useful (albeit professional) mediocrity - meaning they can add a spark OR can be counted on to do certain things
TJ Warren is streaky. If he was capable of being the amazing version of himself every night he wouldn't be a bench player. Ross is also already everything he's going to be and no amount of coaching (this late in their careers) is going to change that
And Ross is just as good as Warren in that aspect - Ross can shoot the lights out as well when he's feeling it - so this "definitely the best" OPINION of yours would only be correct in certain games
Coaching isn't easy. The same strategies don't work in every game. The same bench players don't produce in every game, especially when different match ups are involved. Sometimes offense isn't the answer, which is primarily what Warren brings to the table and he doesn't even bring it every night. Love me some Tony Buckets, but we won't see him very often, it's more likely we'll see TJ Warren