A lot things can happen in 2 years time. He might get injury which is no surprise for full backs. Some new stars may appear. Some established full backs may want to change the scene. Cucurella is not a home grown player, little hope to play in this coming world cup if he stays, age nearly 25. And full backs is a position where UK usually produces good players but it just happens that all good ones are already grabbed by other big 6 clubs(because we wasted time in Mendy) and the next generation like Thomas and Mitchell are just a bit too young for a CL level team. But in 2 years time they may be better options for us.
So here we are. Brighton should sell this summer and grab this opportunity. I will say a price around 40M is reasonable to both sides.