Given how the older FF games are being repackaged/remastered and re-released and sold on multiple platforms for decades now, I see the eventual FF7 trilogy be an evergreen going on for years to come.
They need to make the third entry no matter what, but could downscope it a bit if they don't want to spend $150m+ on the finale. SE has shown it can work on 2 FF games at the same time with FF716 and Rebirth, so I don't think working on FF7 part 3 will delay a FF17 title.