To me the question is a serious one.
who says it isn't? equally the question can only be seriously attended to if the definitions are all as clear - confusion about standards leads to confusion about doctrine - and certainly leads to disconnects about capability ergo relevance.
The M551 design, even if its missile guidance worked, seems to be flawed on several levels, most of all in the light tank role as I understand it based on the time when light tanks were a functional part of how armoured forces operated.
Its the grandaddy of LAHAT based delivery. whats wrong with LAHAT? Its not for everyone, but its OK for some. Its all an issue of relevance. Current gen light/medium/main battle tanks are all LOS platforms or linear weaps delivery platforms. PGM from the barrel were positively archaic tech in the 70's. the same tech today can offer a bit more accuracy and a bit more flexibility. NLOS is probably a better and more flexible capability, but thats been shunted for the moment.
again - its doctrine, use and relevance.
What is important to me, is how classification affects decisions made in ADF and the Army in particular in making major equipment purchases. I can accept the utility of the M1 NOW, but the future for operating heavy tanks does not look good, and come 2025 there will be a questionmark over M1 remaining a viable part of the Army's fleet with advent of the LAND 400 based DF AT platform.
In 25 years time the ANZACS will be ready for the knackery as well - so whats the issue? Any platform at 25 years has probably transitioned through a generational change in doctrine. Tanks went through a partial doctrine change in 1991 - there's no reason as to why they can't go through another change in relevance
capability is about relevance married to doctrine and force structure. who's to say that in 2032 that tanks are NLOS/VLS with far more co-op control than is even remotely possible now?
for years people have been predicting the end of the tank, O'Dwyer was predicting the end of the machine gun as we know it, LeMay was predicting the end of rockets over manned bombers, the poms were predicting the end of manned aircraft over missiles etc etc... all have been grossly wrong and somewhat cavalier in their assessment of the end of a particular platform cycle.
I don't see tanks dying, I see them evolving into different denial weapons - and still potential intruders for the force de main.
assets don't necessarily die, they evolve within the system. looking at platforms in isolation as far as I'm concerned is loading the bases for a preferred outcome.