ATGMs are a whole 'nother breed of cat but the chobham armor is designed with HEAT rounds in mind. FYI the rifled cannon HEAT round is not as effective as the same caliber smooth bore HEAT round due to spinning distorting the penetrating cone. SAGGER was considered a stopgap weapon as the operator had to see the missile AND target. He had to fly the missile into the target- that was found to be difficult to do under battlefield conditions. It took dozens of missiles to score a hit. The SAGGER warhead can not penetrate the M1A1 tank's armor except in the rear. Same with the RPG. The same can not be said about a Stryker or M60 MBT. The Israeli M60 tanks were knocked out by the RPG/Saggar teams in '73- the Abrams was not by the RPG in '91.
Back to main guns- the original fitting of a 10.5 rifled cannon in the M1 was NOT because it was within a curly hair of the 12cm smooth bore cannon in performance- it was because the armor command was quite hide bound. However running the 10.5 Abrams beside the British Chieftain and the West German Leopard2 showed the vast superiority of the 12cm smooth bore over a 10.5 rifled cannon.
I guess you never read about the 3rd AD in WWII Europe when deciding what determines who wins and who burns alive in a tank battle. The 'best' way to fight the Panther or Tiger tank was to have a company of Shermans keep the panther/tiger busy by blowing up infront of it while a plt of tanks attempted to sneak up on the German from the rear.
Given the new computer assisted sighting systems, the desert is a great place for a 12cm chobham armored MBT to pick off a homogeneous steel 10.5 rifled cannon armed MBT at will.
Like I said the 12cm smooth bore cannon has twice the eff range and mother of gawd energy compared to the rifled 10.5. I guess you don't know much about the development of the 10.5 the M60 used- it is a British design we pay a licensing fee to put in our tanks so it was 'letting them make some money' off us as well.