I’ve often wondered why outside backers and ILBs aren’t more interchangeable. I mean, safeties take that ILB role in certain packages. Seems like you’d just want to get your best talent on the field.
If we are talking Edge players at a traditionally OLB spot vs. a true off-ball-LB at a traditionally ILB position, I think they represent two very different skill-sets.
Personally, I no longer think of the traditional defensive positions of DE, DT, NT, OLB and ILB/MLB.
I think the game has evolved and defenses have changed, with a 5th DB on the field more than half the time.
Defenses are much more of a 4-2-5 alignment, if you consider the Edge players DL; or
a 2-4-5 alignment, if you view the Edge players more as LBs.
If a run is expected, there will be an extra DL and 4 DBs but that's less than half the snaps.
T.J. Watt is probably considered more of a OLB because he is mostly standing, while Myles Garrett would be viewed as more of a DE because he usually has his hand on the ground. However, they are in the same spot, with almost identical responsibilities ... set-the-edge on running plays and attack the QB on passing plays (Watt may occasionally drop into coverage). I view them both as Edge-players.
D-Line is more complicated because you have different Technique designations (0T, 1T, 2T, 3T, 4T and 5T) tied to where the DL sets-up and his responsibilities. However, lots of current DL are positioned differently throughout a game/season; and I don't think many fall into just one category.
I don't think Fred Warner (great off-ball-LB) can do what T.J. Watt does ... or, vice-versa.
Just my two cents.