Better, and more imaginative, projectile designs and extreme velocity more than makes up for the relatively diminutive (compared to a 16 incher) projectile of the 120mm Rheinmetall gun. At this point in time, the 120mm gun is limited by the expansion rate of gun powder/propellent. With muzzle velocities of 5,200 to 5,700 ft/s (a mile per second plus), the 120mm gun doesn’t have rifling because that would slow the projectile down.
Doubling the weight of a projectile doubles it’s kinetic energy. Doubling the speed of the projectile quadruples it’s kinetic energy.
Momentum (weight times speed) is also important for penetration but burning a hole thru 20 inches of steel allowing shards of depleted uranium or tungsten to wreak havoc on the other side is pretty effective also.
If you need more destructive force, use a missle or call in an airstrike.
Also, accuracy falls dramatically past 10,000 meters, even in calm seas and cruising at less than 20 knots on an even keel. A number of battleship engagements in WW2 started at long range, aided with radar, but as the fight progressed, the distance shortens and both sides invariably switched to visual sighting.
Several European navies have experimented with this. Germany put leopard turrets on a small destroyer/cruiser. Most applications are for riverine/brown navy uses, where range is not the issue. For the US it would be fantastic for amphibious ops since it would return offshore fire support to the Marine Corps arsenal. Most folk don’t realize the navy has slowly eliminated the offshore capabilities of it’s vessels. since the mothballing of the battleships, there are only a handful of large calibre weapons available to Marines ashore. Using smaller tank turreted vessels would eliminate the concern the navy has for placing it’s high dollar vessels at risk to shore fire, etc. Also, since they would be in close to the beach, the range issue is moot, and target acquisition can be handled onboard if necessary. Of course for brown water navy it is an obvious solution. Cheap, readily available tubes, and ammo, with plenty of techs/mechs around to support. Then of course you have the self-propelled tubes out there in mothballs, and artillery tubes as well. In this age of high deficits and reluctance to spend for defense, these could all be viable options.