Incorrect I think. FFX is regarded as popular as VII now. Based of the latest sales data.
FFX series 20.8mil shipped as of the last update - Sept 2021.
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FFVII original 14.1mil sold as of the last update - 31 Jan 2023.
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Obviously difficult to directly compare popularity, given the X series sales includes X-2. However VII original has also been out for longer. I know plenty of people who regard X as one of the greatest games ever made.
No ones really dropped the series, in fact the opposite can be argued that the series has retained a large amount of its audience, which is unusual for a 35 year old franchise. FFXII and FFXIII sold well I don't think we have numbers of FFXII including the Zodiac age. FFXIV one of the most popular MMOs ever. FFXV gradually sold over 10mil copies. FFVIIR likely at 7-8mil sold now despite being free on PSN 12 months after release and not being on Xbox. The price of VIIR has remained very high too.
The problem with FF popularity is that it hasn't grown, which is a concern as development costs increase so you get to a point where 5-6mil units doesn't cut it. XVI is another attempt at breaking this stagnation, which is ceilings of 7-10mil copies sold and breaking into the mainstream, which is now around 15mil copies or so.