I see your point but disagree with Turner's casting. The goal were young fresh faces for Jean, Scott Summers/Cyclops and Ororo Munroe/Storm and they achieved it mostly (in my opinion) by adding Turner, Tye Sheridan and Alexandra Shipp. All three played their roles well in my book.
Where I would agree is in Ben Hardy poor casting as Warren Worthington/Angel. And a more well-known actress in Olivia Munn, for me, looked absolutely perfect for the role of Psylocke but conveyed very little depth for the character on-screen. I thought neither added strong enough chemistry within the acting mix.