>>64380513 (OP)
As long as gun-armed tracked vehicles remain relevant in war, the M60 will never really be obsolete. Almost all the advances in military vehicle tech over the last 50 years have been electronics, ammo, and active protection, and all that can be retrofitted into an older chassis just fine.
The only real threat to the future of the M60 would be a major shift in what a combat vehicle looks like. If the paradigm shifts from a turreted gun to a module of VLS cells, then it's over for the M60. Likewise, if we shift to railguns/plasma cannons/whatever, it's over for M60, since it couldn't power an electricity-hungry weapon.