To be fair, the RGB likenesses were only on case studies which in the end were never used, and the only appearance of a RGB design was an injoke at Egon's hair.
Now, of course, I could be wrong on the whole ‘GB II rights’ issue, but either way, 88 MPH isn't intending to do any form of an adaption of Ghostbusters II according to what they've been saying, and while this does come as a dissapointment…to a degree, I think they could probably add a better spin on it…but I can more then understand their not wanting to subject themselves to a fixed set of future events.