Coutinho may or may not be Barça/RM material, but he isn't a natural Iniesta replacement. They're different type of players, and Barça don't play with the type of AM that Coutinho is. A better comparison would be Rafinha, and he's struggled quite a bit slotting into Barça's midfield as a CM, and not just due to immaturity/inexperience or a lack of talent. Even if Coutinho was "already developed" and even if he showed promise playing as CM in the Prem, this doesn't tell much - see Fàbregas. The PL and la liga have different requirements for CMs, and in Barça's case this is even more extreme.
Of course he could grow into it with time as he still has room for development, but instead, Barça could just go for one of the better alternatives out there who'd actually be more suitable to the role and who'd probably improve the team immediately. Some of them already even play in la liga. And even if midfield roles were to change, Barça need a regista-type of CM, not another AM, for now.