marcus is one of those guys that got drafted way higher than he should have. he was/is definitely a top 20 pick, but not a top 10 player in a draft imo.
I believe if you're drafting in the top 10 you should be getting an all-star. Marcus is/was never going to be that, I've said from day one he'll never have the numbers to be. all the players coming out of college are projects of course but some are more of a project than others. for example look at the last few picks the suns have made, all those players are massive projects and i'd be willing to bet 2 of 3 will probably be complete busts or never sniff an All-star level, they'll likely never live up to their draft status. too many teams make this mistake.
Yes Marcus is a role player and he never should've been drafted that high. with all that said he is unique, he has the ability to influence the outcome of games and in the end the only stat that really matters is the W-L column.
so is he replaceable? it depends on the criteria, if you drafted him with the expectation of being an all star who averages 20 pts and 10 assists then he's a complete bust if that's the case and definitely replaceable. if you acknowledge that smart is unique and not you're average "role player" then he's not replaceable.