Cannons will not be removed.
I agree, they are useful in real life as a backup when missles miss and aircraft become very close and engage in short-range combat, manuvering around each other, From what I've heard in the past when fighter aircraft get that close they can actually be much safer than missles, which do have a minimum range.
I also seem to remember wacthing footage several years back of real aircraft using cannons on ground targets, although how common such usage is I don't really know as I'm not an expert on the subject. I saw in the videos cannons being used on both vehicles on the ground as well as individual people, although I don't know off-hand if they were the same type of cannon, and again, I'm not an expert.
This may actually be a good addition to the game, as last resort calling in airstrikes on targets if a ground mission seems too difficult. It would only work for some maps, as some tactical parts are not out-doors in open areas (for example, the mine map), although I'd imagine it would be easy to add a flag or variable so that some maps could have an airstrike available (as last resort) and some could not, and I'd imagine it wouldn't require re-making any existing maps, it would just be attaching a variable along with the list of maps, and it could be reflected in the text in mission briefings/loading screens. Just an idea!
As for the things that were in-between missles and cannons, the new technology, I remember they were smaller than missles (I think several dozen could be loaded in an aircraft at once), but they were larger than cannon shells and were fired from some kind of laser-guided device for accuracy, and punched holes in targets via intense kinetic energy force, but I don't recall if they actually had any kind of explosive warhead or not. Again, I'm not fully sure about those points, I'd have to do some digging and research, although they might be a nice addition to the game. I think those things also had a much longer range than cannons, although not the many miles that missles would go, but again I'd have to research it.