Well, all this is merely an exercise in opinion after all, right?
In my view, the Muscle Car era ended with the demise of cheap gas, an explosion in insurance rates, and a general drop off in sales. Most of the last muscle cars languished for months on lots waiting to be sold. There are numerous reports of dealers taking the huge wings off of Daytona's and Superbird's just to sell the cars.
So, when the '73's came out, the Muscle Cars were dead as far as I was concerned. Anemic horsepower ratings were in abundance and trim packages (i.e., the screaming chicken on the Trans Ams) were replacing the "Go" factor with "Show" factor.
Sure, there was a S/D here and a Javelin there but the GTO's, Roadrunners, Chevelles, and 4-4-2's of old were no more.
Yes, there have been as quick or quicker cars built since then. But to me - and this is just my opinion - the TRUE Muscle Car Era ended in 1972.
Being a Muscle Car does not mean it is simply fast or quick. AMG's, M5's, ZO6's and the like are in abundance these days. But they are NOT muscle cars - they are cars with muscle.
The 5.0's, the Turbo Buicks, the TTA's, the LS1 and LS2 F-Bodies - all are muscular cars. But, they are not muscle cars.
Muscle Cars roamed the earth with stump pulling torque, running on 104 octane Sunoco, huge displacements (generally), multiple choices of gear ratios and other performance options and had names like "Boss", "Super Sport", "GTO" and such.
There are quick cars and there are fast cars. But they haven't built a true "Muscle Car" in decades.