Mecha are kin to Starfighters in science fiction. Are they cool? Oh, hell, yeah. Do they make any sense? Not a damn bit. But did I mention they're cool? Oh, hell, yeah.
You can find numerous arguments as to why giant combat robots don't make any kind of military sense. If you have fusion reactors and superlasers for a combat robot, it'll make more sense if you put it in a tank. I won't belabor the details, just hit the high points:
- It's easier to armor a box than a humanoid form. Tanks will absorb more damage than a robot.
- Robots are tall, drawing fire. You'll note tank designers put a premium on lowering the height of their designs. The Russians even had a maximum height for tank crew, 5'9", just to keep the profile down.
- Mechanical complexity kills. Variable geometry wings (swing-wings, like the F-14) carry a weight and complexity penalty that could not be justified, despite the advantages. There hasn't been a new swing-wing to enter service since 1980. Now imagine the complexity of a dynamically-balancing, fully-articulated robot versus a tank with a turret and treads, not forgetting that tanks are already maintenance hogs.
- The g-forces on the larger robots like Gundams would be murder. Animes will show these mecha getting into physical combat on par with professional wrestling. A pilot sitting in a head-mounted cockpit in a 50ft tall mecha that simply falls backwards is going to feel like he's falling off a 5-story building. Even if the robot itself could survive the G-forces, the pilot could not. And we've already reached the point where we can design aircraft who can pull more G's than anyone could survive. As an additional note, air-deployed combat vehicles always have their crew drop separately, whether it's via high-altitude parachute or a low-altitude parachute extraction.
- As stated previously, if a combat robot has a great powerplant and a great gun, it'll be just as good in a boxy tank and better-protected at that.
Mecha vs. tanks, mecha should always lose. But are mecha cool? Hell, yeah. Can we think of examples in military history where rule of cool has kept a weapons system or set of tactics around long past the point at which it should have been discarded? Hell, yeah. Especially when social status and a way of life is tied up in the concept. Can we say landed knight in a feudal society? Of course we can, class.
So, imagine that we've got ourselves a collection of polities we can call Ersatz Europe. Multiple competing countries in close proximity, sharing a religion that has shaped concepts of civilization, honor, and chivalry. Further, let us imagine that significant disputes are settled via ritual warfare with trials by combat, legally binding, agreed to by all parties. While this may remain lethal for the combatants in question, the civilizations involved will avoid the widespread devastation of unrestricted warfare.
Let us imagine that this system has endured for many generations, mecha technology being the pinnacle of military research. Let us assume that the mecha are not unreasonably large, they can be successfully, survivably piloted by humans, and mecha only face other mecha in strictly officiated tournaments. Therefore a country's military is all about building a proper mecha force, of fighting in a tournament. No thought is given to a land army, to an air force, to a navy. All of this is absurd because warfare is between mecha, trial by combat. Nobody has fought with an army since before civilization! Could such a situation arise? Sure. The Meso-Americans practiced ritual flower wars. The Tokugawa Shogunate not only shut foreigners out of Japan, they were even able to remove firearms and keep traditional weapons around for the duration. Barring any significant external threat to such conventions, the only way it will fall is through internal dissolution.
And that's what happens. Sooner or later, some bright leader is going to wonder if he can just do things the really old-fashioned way, i.e. fight an open war. Technology has developed over all that time. What happens when the bright leader says oh, the ritual battle went against me? I have to give concessions to the victor? Make me.
It doesn't matter if what the bright leader has done is unthinkable, it doesn't matter if he's going to be excommunicated from the Church, threatened with the wrath of the gods. He's done it and now everyone else has to decide how they're going to deal with it. What kind of weapons can be created when we are no longer interested in ritual combat but total war? And what of the once-important champions whose specialized skills are no longer relevant?
The story here would be about the mecha pilots. In a moment they have gone from the military elite to useless dinosaurs incapable of justifying their existence. Desperate pilots will try to adapt their machines of romance and chivalry to a form of war that cares nothing for such sentiment. There will be a sense of valiant futility, like the French military academy cadets marching to war in 1914 with white gloves and natty uniforms, destined to be gloriously machine-gunned to death in futile charges in short order. While apocryphal, the imagery of Polish cavalry charging German tanks perfectly captures the death of old romance, crushed in the gears of industrial warfare. One might also look to Takamori Saigo and his Satsuma Rebellion, the last samurai and his fellow romantics blown away by modern firepower.
Could the mecha be adapted into a practical weapon? Is doing so a quixotic deathwish or might they possibly succeed?
My own feeling is that it would resemble Japan's position towards the end of WWII if facing an equally-exhausted opponent. The only pilots left are the very good or the very green. There is no time to train the green ones up to the old standards, just enough to get them in an aircraft and fly straight. It's suicide to even take to the air, even if crash-diving into an enemy ship isn't your goal. But if the enemy is as bad as you are, then there might be some breathing room. The Japanese never had that against the US.
I think that the perfect tragedy would be for the mecha pilots to eventually find a place for themselves on a battlefield filled with tanks and aircraft, albeit in mecha far different from the ones once piloted in the ritual combat. If laser and missile weapons are sufficiently powerful, aircraft might not be survivable on the battlefield. Artillery fire might not even survive defensive laser fire. Small, nimble mecha might serve as fast scouts while tanks represent the decisive force on the battlefield; they persist in a necessary, diminished role. The metaphor here might be birds, the last survival of the dinosaurs, perching in treetops in a land ruled by lions and elephants and gazelles where once tyrannosaurus and triceratops walked tall and proud. (Yes, I know neither of them lived in what would become Africa. Don't ruin my metaphor.)