I feel he may become a CB in the future, especially if you have a manager at some point who plays a back three.
I've already said this but he reminds me of what Azpi was in his prime, unreal 1 on 1 defender (will grow to one of the best around in that regard if he isn't there already) but a bit awkward going forward and that can be a problem in the modern game that relies heavily on fullbacks attacking abilities (even when Azpi was in his complete pomp he was critisized for his lack of forward play), Liverpool the main proof of that, without Trent and Robertson they lose atleast 10 points a season.
I guess another option would be to sign an attacking fullback and rotate both depending on opponent (for example said attacking fullback is the better option against teams like Palace who park a jumbo jet) and will also give a option to go back three with a wingback and AWB playing RCB.
All in all though £50m is a very sound investment given his strengths and likely longevity.