Yeah that is so correct, we already have old tank & modern tank,
having a "future tank" which looks only slightly better than the other two with some better stats would be lame.
To really show "the next big step" comparable to moving from spears to muskets or cavalry to tanks, you really need to push the boat out as far as what would be possible with technology.
What options really did they have, I would of preffered more than simply a new tank like unit, I would have preffered future versions of planes/ships/subs/infantry e.t.c, but hopefully a mod will achieve this.
As far as "the next big upgrade" what will trump tanks in future combat, well their is only so many options and I can gaurentee one of them is not "just more tanks, no technology will ever trump a tank"
Personally the options they could have gone with in my opinion is
- Androids/Cyborgs, man sized combat robots or combat chasis for an infantry soldier, heavy support infantry if you will, who needs large tanks when a man sized robot with a mini-gun/cannon/rocket launcher can do it all.
- Walkers, e.g the GDR these beomoths will be capable of great firepower, from bi-pedal mechs to tripod walkers to insect like 8 legged beasts, perhaps the future will be on large legs to cover ground quickly as apposed to wheels.
- Hover Technology, while some may argue wheels are better than legs, why go back to legs, I can assure you hover technology will be better than both, why touch the ground at all when your infantry/tanks can float off the ground allowing them to travel over any terrain at the speed. Hover Tanks would be a good replacement for the modern tank, however current technology makes flubber like flying cars a bit hard to achieve

.
So what would you go with, the very sci-fi hover technology, the heavy infantry support of man sized "death robots" to replace larger heavily armoured tanks, or the heavily armoured walker.
Both android and hover units would be less armoured, the android because of size and the hover because of weight limitations to be able to hover, a walker however can be armoured heavily. Which makes it a very good tank replacement if walkers can achieve speeds like a tank can, and can be as efficient in combat.