I think the board made the right decision by signing Lammens instead of Martinez. Martinez's Premier League experience is no doubt an edge, but investing in a young goalkeeper with prospect is much better. Martinez is 33 years old and would have cost United around €35-€40 million, while Lammens is 23 years old, that's a huge difference of 10 years and his stats are quite good. I know the Premier League is different from the Jupiter Pro League, but gambling on Lammens is a risk worth taking.
I agree with you, nowadays it's better to invest in Younger players, Martinez is not young anymore and after a few seasons they will still end up looking for another goalkeeper to sign again which we know how troublesome that can be.
Let's just hope and pray that Lammens adapts easily and solve their goalkeeping problems. It's the best choice instead of getting Martinez. Martinez has more experienced and might offer them more quality in post but Lammens is the best choice for the long term. He will definitely grow and also increase in experience with time. We shouldn't always look down on young players, Petrovic was signed from USA league and despite his young age he came in still did well in his first season.