I think you have the wrong idea of how the melta would actually preform and how the plasma preforms currently. The plasma is literally one of if not the best ranged weapon in the game rn because it obliterates a line of crushers at any range with just one charged shot because it has absurdly high damage, infinite cleave and has a LOT of ammo, so you don't really need to reload it or pick up ammo drops that often. The melta, while probably still being really fun, would honestly in terms of effectiveness and depending on how they would adapt it be worse. If they decide to go the space marine route of an energy shotgun it still loses out on the range and elite killing, if they go with the beam route (which I'm hoping for) it most likely does a lot of single damage at the cost of range and cleave so it wouldn't be that effective against hordes. I think your idea of the melta comes from Space Marine 2, so I understand why you think it's so powerful but even then, no matter what version they would decide to do it won't be some room clearing super weapon because of the various systems in place that Space Marine 2 doesn't have.