Yo Alan, firstly, many thanks for the many intriguing and sometimes affable but blunt posts that you’ve educated and amused me with, down the years - all much appreciated.
I think that you’re missing the point that Cancelo is a left back (plays as a left back) but has played right back an awful lot so is also (and maybe originally) a right back.
Football players are players of football, they rock up at a club and the manager says play here or play there and they play in those positions and other positions as may be required.
They aren’t contractually committed to play a specific roll, they’re both expected and allowed to play in any number of positions and to be recognised as being players in the positions that they play in y’know?
Football’s a team game so it’s not about elevating an individual player by facilitating that players highest level of individual performance, it‘s about tuning the instrument to get the best tune out of it.
PS. I’ve just reread your post and no, you didn’t miss a whisker.
Please forgive my clumsy response.