This isn't new news, only a surprise to anyone who's not flown with BA before, as BA is unique in charging passengers to choose seats even in Business Class unless you have status. Also, I think you can only get free seat selection 24 hours in advance i.e. at check in, not 7 days unless you're a Bronze/OWR member.

I'm sure that there are lots of variables that go into seat bumping and having a seat assigned doesn't provide a guarantee. I would just say be one of the first to check in online as that's the best you can do.