I don't believe they were suppose to replace the AR for general issue. SCAR stands for Special Forces Combat Assault Rifle and was made for USSOCOM and other specialists while the Army was looking at the XM8. This SCAR program also birthed the XCR and the ACR was based on SCAR concepts (Multicalibre rifle systems, AR ergonomics, light, synthetic, modular or just basically tacticool). For forward lookers, it was also made when the 5.56 lethality debate was at it's height and if the US forces made a switch to, say 6.8 SPC, it would just be a barrel swap. Also, the ACR was designed by Magpul to overcome what they thought were flaws in the AR design.