As an Army that has to send its tanks around the world to Eurasian battlefields to fight, the Army is (understandably) always tempted by the siren song of "light and survivable."
Yeah, good luck with that:
A lighter weight, more high-tech tank will allow for greater mobility in the future, including an ability to deploy more quickly, handle extremely rigorous terrain, integrate new weapons, cross bridges inaccessible to current Abrams tanks and maximize on-board networking along with new size-weight-and-power configurations.
Although initial requirements for the future tank have yet to emerge, Bassett explained that the next-generation platform will use advanced sensors and light-weight composite armor materials able to achieve equal or greater protection at much lighter weights.
Here's a thought. We find a way to save 40 tons of weight by putting on new armor that equals the current heavy plating that the Abrams has. Voila! We have a lighter tank that can take on the enemy!
Which is nice if we maintain technological superiority over our enemies in armor design.
In the short run, enemies who don't have to build tanks to be sent (and supplied) several thousand miles away build anti-tank weapons large enough to punch through the new armor. They may build the tank large enough to hold such a weapon without worrying about the need to ship the tank around the world.
And if the enemy manages to equal our armor technology, they simply load more of it on their tank to make their tank even more impervious to our weapons than our tanks are to their weapons.
Face it, every time we save a ton of weight we will need to add that weight back on--and still have to endure the strategic mobility problem. The Army needs to build a tank that can survive and win; and let the Navy worry about moving and supplying the damn things.
And even active defenses will not save us from needing sheer bulk of passive armor given that there is more than one way to skin a cat in this survivability problem.
The appeal of building a light but survivable tank lives on. But the wonder tank will not be built (see "Equipping the Objective Force")
Oh, and I issue a hardy laugh at the idea that a faster tank is more survivable. Please examine British World War II experience with their cruiser tanks. Speed never replaces armor. No tank will ever be faster than a shell or missile.