Hard to think that Kopitar isn't an excellent Captain. The question more is do the Kings have the right on-ice leadership mix and is Kopitar the right Captain for the Kings today (including how he works, and potentially balances, the leadership style of the coaching staff).
Brown, in my opinion, is the best Captain the Kings have ever had, but would not have been as effective without the right supporting leadership cast (those with an "A" like Kopitar and Greene, and many leaders on the team without one {Stoll, Williams, Mitchell. Gabby - just to name a few). Kopitar was an excellent "A" on that team as his style, among other things, helped to balance out Sutter's.
An excellent leader isn't always the right leader for the team.