There is a great deal of confusion between RPG and rifle grenade, most of it caused by the media. An actual rifle grenade, that fits on the end of a rifle, only has about half the range and the same explosive force of the M203. This was the reason for the invention of the 40mm in the first place, to get longer range over the rifle grenades that were then in use. Over the same range the 40mm will have greater accuracy.
Even today rifle grenades are used by many nations, but most of them have an effective range of only about 100M or so. There have been R&D programs that show promise, but The USMC already IS using a version of the RPG. The SMAW is based on an Israeli weapon that's based on the RPG-7, so the Marines already have a weapon that is basically the same as the RPG, only better.
While those Marines are expressing a valid concern, the rifle grenade isn't the solution for this problem.
Ross