It is more of a gut feeling, and kinda has to be given both Reke and Marcus are 2 years in. I watched Marcus a fair ammount while with NO last year, mainly because I like watching CP3, and keeping tabs on Peja. In what I saw last year, as well as this year with us, my eyes tell me he's a natural high scoring 6th man. I'm just not sold on him being a fulltime starting guard, especially next to a guy like Reke, who up until this point thinks shoot first.
I didn't see Reke and Marcus playing off each other as much as you did I guess. I saw them competing for shots, with Cousins as well. I saw a reluctance from both to play inside out, or even give Cousins the ball on nights he was rolling. Maybe that will change after a training camp, or a more structured offense is implemented. I'm not sure. I'll let the powers that be decide.
I like Marcus a lot, and if Petrie and Westy decide they are our backcourt next season, I'm fine with it. Maybe I'm completely off point here, and after an offseason and full training camp, they'll mesh better both together and with the team, and they'll solidiy the backcourt for years to come. But I'm not completely confident they will, at least not the point we don't take an Irving if available, or even consider adding a point. With them I also see two young natural scorers, who want to make their mark in this league. Will they both be able to temper that enough to help the team? I don't know.
I'm nervous about having a starting backcourt where neither player looks comfortable running a team. Yes I do think Reke could get there, and he'll have the ball in his hands a lot no matter what. That's also why I'd never pair him with a Rubio/Rondo type, because they're both worthless off the ball. I've never been high on the idea of having two 20+pt scorers in the same backcourt, and even moreso with arguably a top 3 offensive center on the team, who's also arguably our best playmaker/creator at this point.
Its only my opinion, but down the road I think Reke will be best with the ball looking to score or create for others, with someone next to him who can hit the open shot, but doesn't require shots, and can create for himself or others when need be. I also think Marcus will be best in a role where he mostly looks to score. For both Marcus and Reke, their strength is scoring, and the more we try to turn them into creators, the more we take away from their strengths, and the more they look to score, the more they take away from Cousins.
Its not the same, but I'd compare it a little to OKC, with Harden, Durant, and Westbrook. Harden could start for them, and is a very good offensive player. But with him in the starting lineup, it might take away from what Durant/Westbrook do. Coming off the bench really allows Harden to play his game, and all 3 end up having a larger impact on the game. As Cousins/Reke mature, I see having another guy looking to score20+ in the starting lineup taking away from their games somewhat, whereas if Marcus comes off the bench, he can do what he does best, score, without affecting any chemistry between Reke and Cousins.