This is true. You're trying to replicate the "old NFL" where teams mostly threw the ball to the two outside WRs which is what Cover 2 was designed to take away since you have a corner and safety on each side. In the modern NFL, teams often attack from the slot, tight end and throw over the middle far more often than they used to. This was the result of rule changes due to player safety. As mentioned above defenders can't deliver the same knockout hits as before.
It doesn't make much sense to ask your Mike LB to defend the deep third and play the run at the same time. It's the weakest zone defense against the run. On top of that, the shallow to intermediate middle is open if he does sprint back to cover the deep third. That's an easy completion for any QB.
Nonetheless, Cover 2 still has value but in terms of a "trap defense" Cover 4 Palms being the best example that looks like your modern Cover 2 trap defense. Teams just don't base out of Cover 2 anymore.