That's not entirely true, there was intermediates of those weapon systems.I'm not sure why you're arguing that it can't be successor to the T51 and T60 because it's too radical a change in looks. We went from the M14 to the M16, and the M60 Paton to the M1 Abrams, which were fairly radical shifts in terms of looks, function, etc.
America didn't skip M60 straight to M1 Abrams, they had the MBT prototypes, XM1, and so on.
The MBT prototypes actually kinda look like a mishmash of M60 and M1 Abrams as well. ;D