Thucydides said:
If anything, conventional tank gun artillery is about where modern rifles are; at the peak of evolutionary development. The costs of pushing for another few percent improvement in performance are far more than can be justified, and when you consider that even the classic L-7 105mm cannon from the Leopard 1 hits the target with 13 million foot pounds of energy, I suggest no modern tank will drive away from a kinetic energy strike from a 120mm round in any condition to keep fighting.
I respectfully disagree. Tanks can and will shrug off hits from other tanks. M1A1s whacked each-other in the face with Silver Bullets a few times, no damage. That was during the Gulf War, before they had the DU armour, or the ERA. How'd they survive firepower of that magnitude? Composite armour.
Modern tank armour is weird voodoo stuff. It's a composite of steel, ceramic, aluminium, U-238, rubber, explosives, plastic, and some other stuff besides (tank dependent). We talk about its effectiveness in mm Rolled Homogeneous Armour equivalent (mmRHAe), how much armour plate you'd need to have the same protection. It's also differentiated for attacks by kinetic projectiles, or attacks by shaped charges. Most armour (especially when ERA is involved) is drastically more effective versus shaped charges than KE. The exact numbers are (obviously) not published, but we have some good estimates.
The hull and turret front of a T-72A varies from ~300-550mm RHAe. As a comparison, this:
is the citadel of USS New Jersey, 17 inches of solid steel (which, *25.4, is about 430mm). The T-72A, a decidedly mediocre tank, has as much armour as a battleship.
What about something a little more upscale? Leopard 2A4 has 750mm vs KE on the turret. Challenger 2 is reckoned to have a whopping 1250mm vs. KE, and as much as 2000mm vs. HEAT on the cheeks. Armata is "supposed" to have 1000mm on the front.
Now, to counter this, we have absolutely massive guns. The Rheinmetall 120mm/L55 puts out about twice as much energy as the 105mm L7 (designation, not caliber). Throwing DM53, it'll penetrate maybe 800mm RHAe (again, estimated, not published). That's almost enough to go in one side of that battleship and out the other! Still not enough to penetrate a Chally 2 or an Armata (maybe). Hence, the need for a bigger gun. Despite your assertions of only few more percent, Rheinmetall believe they can get 50% more pen out of the 130mm/L51 (though, of course, I'll believe it only when I see it).
Thucydides said:
Short term I suspect the trend will be to 55 calibre gun barrels and a mixture of advanced APDS-FS rounds for close in work and "smart" rounds to allow tanks to extend the battlespace (K-STAM has a range of 8 Km and LAHAT can fire out to 13 km), and HEAT-MP for anything which isn't a tank.
Following on what Nerf herder said, missiles ain't that great in a world where everyone has an APS, laser detectors (sucks to be you, LAHAT spotters), jammers, and smokescreens. Can't jam a big angry heavy metal dart, though.