Matic would be an excellent fullback! As I think would Hamann have been.
People are too allied to "positions", but positions don't really exist, only skills and athleticism, suited to the area of the field. That's why teams who do a lot of small sided games in training always have flexible players and formations, because once you go to 4v4 or 5v5 or something like that, positions stop existing, and there's only "where should you be in this moment?".
It's like "Total Football" didn't teach anyone anything
I agree with your wider point that we're too fixated on positions (and have long felt managers should experiment more - it's something I really liked about Rodgers!), but on this specific discussion it feels like you're under-estimating athleticism and over-estimating skills a little.
Matic certainly has some of the skills to be an 'excellent' full-back, but he lacks the fundamental athleticism to be a sustained success there at an elite level IMO. Could you imagine Mané or Oxlade up against him on Saturday with space to run into? Sure, his excellent reading of the game would help, but eventually he's going to get burned.
Going forward, too, I think he simply lacks the pace and agility to be effective on the overlap.
I think your diagram on the previous page would have been accurate ten years ago, and it probably still is at lower levels with more defensive-minded full-backs, but it's moved on at the very top now. Generally, full-backs' average positions and general skills are more in line with a box-to-box midfielder's than a holding midfielder's.
Of course, it's all player and system dependent though
(As your reply to me which I've only just seen basically summarises!)