As time goes on, the technology will become cheaper and more easily available, but you STILL NEED HEAVY ARMOR FOR THINGS LIKE RPGs and such. You
CANNOT make lighter, "more survivable" tanks right now, because lots of armor is one of the keys to survivability. You don't drive a 25 ton armored
personnel carrier into a heavy firefight like a tank; it'll get chewed up by RPGs. A 70-ton Abrams, employed correctly, is a match against those
things. Until armor becomes lighter, but stronger, tanks will remain at 70 tons.
Also there is the thing called mines. An armored Humvee that drove over a mine got blown about 10-15 feet up in the air; the crew was okay though
because it was an armored Humvee. But my point is, even if you have a light tank with armor making it as strong as a 70 ton Abrams, if you could
develop cheap mines that could knock the tank upside down, it's kinda useless.
Yes, the engineers are supposed to go in and get the mines out first, but there is always the chance they'll miss, and I'd rather be driving a tank
where if you hit a mine, you just have to replace a track and make a few repairs, rather than one where if you hit a mine you could get turned upside
down.
In the recent Iraq War, a lot of the tanks and vehicles were on narrow roads with ditches on the side. You get a light tank that hits a mine, and
BOOM! you go straight into the ditch and the crew drowns, even if their armor protected them from the blast.
Heavy tanks I see staying for a long time; if anything, since infantry weapons will improve, the newer MBTs will probably have stronger armor, but a
lighter, more powerful engine, thus allowing even more of this stronger armor, so the tank can weigh 70 tons, but be more withstandable to weapons.
I don't see tanks going past 70 tons, mainly because there is a limit on what the modern aircraft that transport them can carry. The limit is 70
tons. Sure, you could build bigger aircraft, but that costs a crapload of $$$, and also you'd need some big-ass runways for it, and in wartime,
runways are never the ideal length. So tanks will remain at 70 tons, no heavier I'd think.
As for the turbine engine, it has one slight problem. The heat signature. In Vietnam, troops could move directly behind the M60 tanks, whereas
nowadays I think that is kidn of difficult, because an Abrams puts out a lot of heat, so you can't be right behind it. What they need is a cooling
system I'd think, or a way to reduce the heat signature on the turbine. Otherwise, the turbine is better than the diesel. But that heat signature
makes it vulnerable to enemy aircraft in a future conflict, or enemy infantry weapons.
Oh yeah, you say the U.S. needs something that it can put at the front lines immediately until the heavy stuff arrives?? We've had that for over 100
years. It is called the United States Marine Corps