What happens if we pay 45-50m for Cucu and JWE turns out to be a highly talented LB who wants to play much more minutes?
If JWE competes with a cheaper new LB, he can displace him and play gradually more and more minutes. But if a 50m LB stands in his way, his playing time will stay limited even if he improves a lot.
From JWE's perspective, it's not the same whether we buy a back-up LB to rotate Cancelo or a first choice LB. An expensive first choice LB would limit JWE's minutes over the next 2-3 years much more than a squad player like, say, Sosa.
This is probably a minor point, but it might have some weight for Txiki and Pep.