That is much more harder than it sounds.

Weight savings matters to the customer, but a big part of the cost/benefit equation from the airframer's perspective is saved labor. The design/testing costs for a composite wing are about the same regardless of wing size. If you're going to build widebody wings, that's a lot of labor-work saved over the life of the project... not as much for an RJ. To get an ROI on saved labor you need to either build large wings (lots of spars, rivets, panels, holes drilled, etc) or build a whole lot of smaller wings... a niche RJ isn't going to provide that opportunity although you can bet your arse the next generation NB's (on both sides of the Atlantic) will have composite wings.

Also spending money to save weight is easier to justify for longer-range aircraft... once they get up to cruise the saved weight and smoother surface, and cruise-optimized wing shape amplifies into fuel savings over the duration of the flight. Widebodies (not coincidentally) are the primary market for composite wings now. Smaller planes which are up and down a lot don't get the same advantage, since they spend a lot of time in transition and on the ground. That costly, light, smooth composite wing is a liability at the gate and in the departure lineup at the hub.

You could do composite just to get to a specific weight goal, but that would likely be an economic fail. Better to just design your plane to be at the correct weight from day one. MHI was almost arrogant over the years in their disregard for US scope, and I for one am enjoying watching them squirm a bit (OK a lot). Message sent loud and clear.