The inside linebacker isn’t dead. There’s just fewer of them needed to defend the most popular offensive personnel packages. NFL offenses are taking over 70% of their snaps in one-back formations with the immediate threat of 4 vertical receivers.
Against these personnel groupings, defenses usually have one true middle linebacker (Mike) who aligns inside the box against most formations, and a second inside linebacker (e.g. Will) that often lines up outside the tackles and folds inside on runs. Because the Will requires more speed to the sideline and makes more tackles in space, he’s more of an outside linebacker type.
But Mike isn’t just an A/B gap run fitter, and it’s possible that the escalating salary cap cost of elite defensive ends will force NFL defenses to feature the Mike in their blitz packages and rely less on edge rushers.