Red 6 said:
AT missiles provide a lot more versatility.
AT missiles are special purpose munitions, and are brilliant for taking out a hard target like a tank or a bunker. A cannon provides the ability to switch rounds (i.e. smash a bunker with a HEAT round then follow with an HE or smoke round against the exposed enemy), and a much higher rate of fire. Even a Starstreak takes about 5 seconds to reach it's maximum distance of 6 KM, and it is under operator control the entire way. A TOW takes about 23 seconds to reach out to @ 4 KM, which would give a tank or other gun crew time to fire 2 or 3 rounds.
Guns are complimentary to missiles, the example of the WOMBAT was incomplete; the WOMBAT operated as part of an integrated fire plan with the MILAN anti tank missiles, using its high rate of fire to suppress both enemy APCs or IFVs (if the Russian MRR was equipped with BTR's or BMP's), as well as being able to fire at dismounted troops (the huge 120mm HESH round would be quite effective in creating a huge shock wave and throwing debris like shrapnel when hitting the ground), something you can't do very well with a missile.
There were lots of reasons the weapon went out of service, I suspect a larger reason than any technical arguments over effectiveness was the declining manpower base of western armies over the years, a missile crew is generally smaller than a gun crew, and as manpower ceilings were slashed it was judged more effective to concentrate on the missiles rather than try to maintain both guns and missiles.
Recoiless rifles using the countermass principle do have some pretty big limitations; inefficient use of propellant (4/5 flows out the venturi to balance the recoil of the shell, limiting range and muzzle velocity), and a huge signature when firing. I sometimes think it would be worthwhile to develop a new High/Low pressure gun which reduces many of the problems associated with countermass weapons. This would provide a lightweight cannon which does not need a tank to carry and provide the benefits of a cannon on the battlefield.