It's not independent of their shirt numbers, though. I can remember the uncertainty before both Lawes and Itoje started calling lineouts internationally; neither of them did it regularly for their club and it was seen as a risk for England when they did it. Both of them were successful and it's broadened England's options.
I remember the same. The problem here is that lineout calling isn't tied to the shirt number; it's tied to how good they are at understanding lineouts, predicting opposition plays, how strategic they are with their thinking, etc. Sometimes even a flanker (like Tom Wood) can be a lineout caller. It's another piece of the puzzle to juggle.
Likewise, I see Kruis as a direct successor to Kay because they were both setpiece specialists. Kay might have been a bit lighter and faster but he wasn't exactly an all court athlete as England learned to their cost in the RWC Final. On the other hand, it was only in the period between Marler and Kruis coming on and the Boks deploying the bomb squad that England had anything approaching an edge in the scrum at the RWC Final.
I get where you're coming from but Kay was more of an all court player for the time than his drop in the final suggests! Agree about Kruis and Marler coming on in the final, but that's an easy one: Kruis was our biggest scrummaging lock, right? Which suggests he's more of a "4" in your parlance. And he came on for Lawes...
Things were a lot more clearly delineated back in the day. In the Johnson era, it was common to consider your grunt lock the "2" jumper in the lineout - the stronger, less mobile, banker ball if you threw it straight at him - and the other one the "4" jumper who had more lineout ability and mobility. Lineouts were much less sophisticated (players basically stuck to their positions in the lineout) and back rowers nowhere near as good (or as big!) as they are now. These days most sides can rustle up 4 genuine lineout targets and every single one of them is likely to be miles better than the majority of locks of any type from back in the day, so the sharp difference between 4 and 5 doesn't really exist in the same way.
I think it's useful to have archetypes but that only goes so far. Clearly teams can be successful picking monsters at 4 and 5. The old adage of "they're too big to lift" just doesn't seem to matter any more, there's more jumpers available than before, and who you're picking as your lock pairing will depend primarily on a) who your best players are, and b) who makes up the rest of the pack. If you've a massive pack, picking a lineout genius is a no-brainer as you won't lose much; if you've got two superb lineout forwards in the back row, two behemoths gives you a big boost; if you're lacking ball thieves, an Itoje (class aside) or a Beirne is a huge bonus.
Enjoyable convo, appreciated it :)