This in my opinion is what the main difference in "tank" versus "aircraft" really comes down to. It is essentially "area of fine control" - an aircraft can control a fairly large area quickly, but smaller elements can be easily missed. A tank, on the other hand, only controls a small area, but small details abound. Infantry controls a smaller area but they can find the coins on the ground.
I am not quite sure I buy this, at least in Traveller terms. There's still a need for tanks and aircraft today because the missions are so very different and our technology simply doesn't allow us to make a cheap multipurpose unit that fulfills both missions - and both missions still exist.
When you get a fusion powered grav tank, you essentially can build a fast-moving (aircraft) vehicle with plenty of armor and good weapons that can also slow down to a human's walking speed or just stop indefinitely (tanks). A grav vehicle can also gain the advantages of altitude (aircraft) and can even stand in place in mid-air indefinitely (helicopters).
My primary beef with the idea of the "grav tank" is that ... it's not really a tank as we know of it anymore. It's really more of a stripped down or "dumb" starship - it lacks a jump drive, various long-term crew amenities (or it might - advancing technology I think would increase the habitability of this vehicle so the crew can live in it for days or weeks without emerging), but carries a lot more firepower for its mass, and more armor but is ultimately cheaper than a true starship. Grav plates also mean that it doesn't have to worry about mass/ground pressure issues that make lighter vehicles useful. This vehicle has replaced helicopters, armored cars, tanks, and aircraft.
It's not that surprising - it's the same way infantry are no longer divided into archers, pike, and swordsmen. The roles become unnecessary and were replaced by the modern infantryman; there's something to be said for having specialized units for different tasks, but sometimes roles themselves become totally pointless due to advancing technology. (I won't get into riot police because they're essentially warriors fighting in ritualized warfare.)
Grav tanks are tackhammer carrying tacks...
I understand the apparently applicability of this sentiment, but I'm sort of iffy on how it holds up at TL13+. Looking at the Traveller TL levels, by that time there's a lot of very destructive firepower available to an infantryman. The idea of graded firepower is mostly a concern for "limited" warfare where you're usually trying to reduce or eliminate collateral damage, otherwise you try and give your infantry the most effective weapon you can give him. A FGMP already is pretty indiscriminate. A squad of guys with FGMPs is very indiscriminate - I'm already iffy on the purpose of such infantry. If they feel the need or massed FGMPs to fight something, you as might as well destroy it using a meson strike, I think.
So infantry are going to be used for counter-insurgency duties and similar roles where super-destructive weapons are unsuitable - anything really threatening has already been blasted from orbit. At that point, the FGMP already is really a god of the battlefield - it's going to be able to flatten any building we used armored vehicles or airstrikes on today. Any bigger threat is going to be eliminated by the starship (or whatever) that can be sitting 10,000ft above the infantry patrol - far out of range of insurgent weapons but still able to see coins lying on the ground thanks to TL15 sensors and able to hover there indefinitely thanks to grav plates. Perhaps it has a host of weapons ranging from lasers to mass drivers to a meson gun so it can even grade its responses.
But again, I don't think that vehicle will be a grav tank.
Again, I don't question the existence of a grav APC - though I'd imagine that thing would basically be a heavy armored grav pillbox for the infantry to hide in if the enemy
do have something really nasty - hide in long enough for that starship thing to reduce target to dust with enough protection to shrug off enemy weapons. Now, some enterprising designer might put really nasty weapons on that APC so you don't need a separate high flying weapons platform - but again, you have the fusion of weapons platforms at that point and the primary purpose of the vehicle is to be a refuge for infantry, not to support them (FGMPs and TL13 sensors should be enough for that in insurgency warfare). Is that vehicle a tank that carries infantry? Or a really heavily armored APC?