We need Cancelo as a right back. Plus I think he slows down our ball progression when he plays LB due to being right footed. I do think Aké could make an excellent left back but we've not seen much of that yet.
I would certainly agree that it is no longer a priority position, but as I say I wouldn't say no to a "natural" left back if Mendy left. I wouldn't be too mithered if Mendy stayed though tbh, you never know which player Pep is gonna turn around next.
We have two world class RBs - Cancelo is a key cog in this team, but we don't 'need' him as a RB when he's arguably not even our starting RB.
Having him balance his game time between RB and LB is perfect and gives a player as good as him ample gametime.
Mendy is just a non-factor - him leaving this squad will literally have no bearing on selection given how far down the pecking order he is, so a replacement for him even if he does leave is just unecessary in my opinion.