Pique, the thing about OM&M is that most kids enjoy it. It's short, accessible but is clearly an 'adult' novel that does still offer challenge for the more able. Teaching OM&M does not prevent and has never prevented independent readers from reading other stuff for pleasure. OM&M tends not not put off either more or less able kids.
Steinbeck was one of the writers who opened the doors to grown up literature for me along with F Scott Fitzgerald. Admittedly I was much younger and started with 'The Pearl' and not OM&M but then worked through all of Steinbeck, all of Fitzgerald before moving on to Jane Austen and then Dickens.
The surest way to turn kids off literature (or at least school literature) is to plod through difficult texts where you have to explain the vocab and tone before the kids 'get it' at all. This is off putting for less able kids and for more able kids who hate the tedious stop start pace and just want to read.
In short, if Gove thinks Dickens and co will turn people on to literature and broaden their horizons, he's mistaken. Few kids are going to come away from their English lessons wanting to pick up another book by Dickens (sadly because I LOVE Dickens) whereas I think Steinbeck does encourage kids to move on to the next level.