The Machine Gun tree moving from being partial sidegrades to just strictly-better progression was a bit of a dumbing down. In A17, A19 etc. the AK was a fair alternative to the Not-AR-15, but not anymore.
I think if you wanted to make it more of a tradeoff, holding the M60 should slow you down quite a bit (I mean come on, we're talking about a weapon which is normally crew-served, not even like a BAR or something similarly intermediate), this would keep its value as a horde night gun, while making it more difficult to use as some sort of high-power assault rifle as the game does.
The pistol tree does a much better job of staying interesting for most of its incarnations, even though the damage progression is still always upwards. The rifle and shotgun trees don't bother me because those are more situational/one dimensional weapons due to their inherent drawbacks (damage volume vs range). It's really only the Machine gun tree where this stands out.
EDIT: Though for reference the SMG is probably the most viable Horde Night main weapon after the M60. It actually has a higher rate of fire than the M60 and I believe the fully perked DPM is comparable between the two.