Not necessarily, from a story, design and pre-production perspective part 3 is already complete and will be even more straightforward than Rebirth from the sounds of things.
CBU1 is comprosied of multiple staff who have been both directors/producers, such as Hamaguchi, Nomura, Toriyama, Kitase etc and so between them I'd think they have capacity to work on other projects aside from FF7, such as FF17 or an FF10.
Also well before development wraps up for the final game they can start moving over staff to something new.
It's hard to say at this stage, so it will be interesting to see which staff and division ends up leading it.