

Alan Rickman was the absolute perfect casting for Snape. Nobody else could have brought him to life so well and so true to the books.
Of the entire film cast I think everyone could have been replaced except for Snape, therefore it does make sense to be bold and try something different in his casting for the new show.
Perhaps it wasn’t true the books then, but movie Snape was a believable age to have been at school at the same time as Harry’s parents. It’s just for whatever reason the author made the parents ridiculously young