But I just don't buy it as a replacement as an assault platform.
That is true. The V-22 is by no means an 'assault platform.' It will get in and out of an LZ more quickly and quieter than previous aircraft they are replacing and the other alternatives, and as such won't be as much of a 'bullet magnet.' But it is a 'troop carrier,' and not an assault aircraft.
I see it as adding a new capability but not replacing all the other aircraft capabilities that it has replaced.
That is mostly correct. It does outperform the aircraft it is replacing (CH-46) in virtually all areas, but we need to look at all aircraft currently available including new and soon to be produced aircraft. Most on both the pro and con sides agree the 53K is an outstanding heavy-lift, high-altitude hover helicopter that should be brought into the Marine inventory along with the V-22 medium-lift, high-speed performance tiltrotor in order to provide mission specific capabilities with each aircraft performing missions that the other cannot.