Gove has made many bad decisions, but I'm not convinced this is one of them. Yes, To Kill A Mockingbird is a Great Novel - but it's not as if it's been suggested that it be replaced with something that's complete rubbish. It's a shame that kids won't get to read it, but surely that's made up for by the fact that they get to read another work of equal merit instead. Or do people seriously want to argue that the nineteenth century classics in question are, in fact, badly written or irrelevant or whatever? If so, I know who I think are the philistines.
At the end of the day there's only so many books you can fit into a GCSE syllabus. You're going to have to make difficult choices like Charles Dickens vs Harper Lee or whatever. It's just the practicalities of the thing. I rather suspect that if Great Expectations or something had been the number 1 set text for however many years and it was mooted that it be replaced people - the same people - would be complaining just as much, on the same grounds: that teenagers are being denied good literature. But that isn't the case. Thinking maybe we should have another book in its place isn't the same as thinking To Kill A Mockingbird is terrible. It's not as if it's universally considered to be The Single Best Book Ever Written or anything anyway.
The main argument in favour of some of the current books seems to be that they're easier for teenagers to engage with or whatever. Perhaps reading a book that's too hard will put them off reading forever! This seems a bit condescending to me - it's as if it's saying 15-year-olds can't actually cope with nineteenth century stuff, so give them something easier. Likely there are some kids who wouldn't be able to cope - but that doesn't mean everyone else should be denied access to the "harder" stuff. And I rather suspect that if you've reached the age where you take GCSEs and you're still at risk of being put off reading by a book that's too hard, reading great literature probably isn't ever going to be your thing anyway.
There's also, relatedly, the length argument. There's not much time: short books are better. Hence the popularity of Of Mice and Men. And it's true, a lot of nineteenth century books are very long - but not all of them. And it's not as if all the reading has to be done in the classroom, or that every last part of the book need be analysed. Most people should be perfectly capable of reading even a very long book over the course of a term or two as homework.