The problems of the MBT are snowballing. Essentially, to stay competitive with other tanks and to meet emerging threats, the tank has gotten downright HUGE. The biggest problem on the horizon is the top-attack round: just using the dimensions of a typical tank, armoring the top to the same thickness as the front will add the equivalent of six times the frontal armor weight to the tank's overall weight. That is a LOT of deadweight to haul around, and the weight to support it will act as a domino effect on overall weight--100 to 150-ton tanks might be the result. It is ultimately what killed the battleship (even absent aircraft as a significant weapons carrier, the expanding range of naval gunfire would require ever-thicker deck armor to keep out plunging fire, and deck armor is MUCH heavier than an equivalent vertical belt).
Simply not true. The Russian model, as an example, T90 is a very survivable tank and utilizes 3 layers of armor, yet is barely more then 1/2 the weight of Western tanks. The West has gone with the same falacy of Hitler, bigger is better. The Kind Tiger was 70 tons but was a crap machine fit only for light mobile defense work. The Panther was much lighter and a lot better.
The race between offense and defense is nothing new and has been going on since warfare started.
You also ignore the advance of technology. New stronger and lighter polimers are constantly being developed, along with new and improved power packs/plants that make the weapon system more maneuverable. The main survivability feature of a tank is not armor, for that you build forts or assault guns...it is maneuverability, the ability to out maneuver your enemy, beat him to the objective, hit him from the flanks before he has time to prepare, in other words...run the bastard over.
By the way, the T90 has very effective anti missile defenses that the US/Western modils don't even come close to. Or in most cases even have anything in development that's equivalent...and this is already long time in production.