Early signs suggest that Final Fantasy 7 Rebirth will struggle to match Final Fantasy 7 Remake in terms of commercial success. Although admittedly, we don't have much data to work with right now. According to Games Industry's Christopher Dring, who often outlines key sales info, the recently released sequel is down 30% on its predecessor's launch week figures. It did still manage to top the UK charts, mind.
However, there are a few factors to consider here. For starters, this is based on UK boxed copies only, so no digital sales are taken into account. What's more, it's important to remember that Remake launched during the height of the COVID-19 pandemic in 2020, when everyone was told to stay at home. A lot of titles released during that period sold especially well, generating a noticeable increase in profits across the industry.
Generally speaking, video game sequels — especially direct sequels — tend to lag behind their predecessors in sales. It makes sense when you think about it as most people will want to experience a series from its point of origin — but that obviously doesn't stop publishers from pumping millions into marketing campaigns and the like.