As lasers grow smaller and more efficient, and as power sources become more powerful, I fully expect lasers to become a vital part of point defense and space warfare. However, in atmosphere, they will never supplant missiles and guns - although they might be able to render obsolete the really large missiles and guns.
The reason why they'll never supplant missiles and guns (especially not the smaller ones) is due to the inherent downsides of lasers. Fog, mist, rain, and snow all significantly degrade lasers, even extremely high powered ones. Lasers are limited to "line of sight" weapons, and as such, are useless to submarines, artillery, and other weapons systems that (hopefully) never see the enemy.
If lasers render missiles and guns obsolete, it will be in the same way that the USS Monitor and the CSS Virginia (although not the first Ironclads, among the first to be used in anger) rendered naval smoothbore cannon obsolete. Not by being better at the job, but by preventing the job from being completed.
Drones and aircraft will be the first things to go. Drones, especially, are extremely vulnerable to lasers. They are slow moving, meaning that a lower powered laser will have an easier time staying on target long enough to destroy it. Yes, many (most) military drones have minimal radar returns, making it difficult to distinguish them from birds on radar. For this purpose, the MK1 Eyeball and other similar sensors may make a comeback. In fact, it may just be possible for someone to cobble together an anti-drone laser from publicly available parts. 2 Watt lasers already burn through plastics with ease, and given that drones are usually made from plastics and composites due to radar cross section and cost, it really doesn't take too powerful of a laser to pose a threat to drones. The sensitive electronics inside the drones are even more vulnerable than their plastic/composite exteriors, and in the end, some smart fellow is going to create a laser array to get the same effect out of a bunch of cheap little lasers that he would get out of one really expensive laser.
Aircraft, on the other hand, require more powerful lasers if you're going to take them down by damaging critical equipment. This assumes that you're not classifying a pilot's eyes as critical equipment, or that you're simply not quite enough of an asshole to be willing to burn someone's eyes out just to kill them. Unfortunately, there are plenty of assholes out there who are willing to burn someone's eyes out. Some of them aren't even doing to kill people, they just think it's funny. In the end, lasers used against manned aircraft will vary in power depending upon what kind of kill you want and what sort of resources you have. If you have to kill an aircraft and you don't have a multimillion dollar budget, you're either going to be improvising some sort of rocket, or you'll be shooting a powerful handheld laser at someone's eyes.
Point defense, that is, shooting missiles, rockets, and other assorted ordnance out of the sky, is another place where lasers will excel. With a response time limited only by the laser's ability to target and destroy, lasers will come to dominate close in point defense. The old CIWS won't go away - as per the downsides in paragraph 2, they'll need something that can handle adverse weather conditions as a backup - but it'll be relegated to the backseat. Unfortunately, for the warfighter on a tight budget, this is a realm mostly occupied by expensive systems. I'm sure that something could be cobbled together using a camera, a repurposed Arduino or similar, and a homemade laser array, but it would require technical expertise well outside of that of the average person. In fact, it would probably require a small team to create something of the sort.
Space warfare is the true domain of lasers. Without an atmosphere to limit ranges or produce fog, and aimed at fragile, yet important systems, it is only a matter of time before lasers become predominant in space.
Infantry wouldn't have much use for lasers as a primary weapon. Note disadvantages. However, as a support weapon, lasers have much to recommend them. Consider the previously mentioned 2 watt laser. From a review (also, check out the referenced manual):
A 24-page User Manual is included. Six pages cover the hazards of a Class 4 laser. Most of this material was written by LaserPointerSafety.com, so we are a bit biased, but we do think this is useful and comprehensive. Hazards discussed include:Hmm... Skin burns? Flammable Material Burns? Imagine sweeping a group of men across the chest with this laser. You'll either: A. burn them or B. set them on fire. Either result is going to end with them giving you plenty of time to shoot them dead.
The user is cautioned to wear safety glasses. The user is told that this is NOT to be used as a laser pointer (it is too bright), not to aim at vehicles or law enforcement officers, and not to harass or annoy others.
- direct, reflected and diffused beam eye damage
- skin burns
- flammable material burns
- never aiming at aircraft or stars
- blue light photochemical eye damage
Most other purposes are beyond the means or impractical even for the richest of Nations. Anti-tank lasers? Forget it. Artillery? Impossible, unless you think that lobbing really expensive lasers at an enemy is a good idea, or that nuclear bomb pumped lasers deserve a place outside of space warfare. And in neither case do you really have laser artillery. So, in the end, while lasers aren't the future of warfare, they're definitely part of the future of warfare.