I am going to make a perhaps a not so hot take anymore.
FF is struggling because JRPGs are thriving. The sheer amount of quality JRPGs in the past few years has been astounding, and FF has to compete against them all. Atlus JRPGs have seen quite a bit of growth. Sega has joined the mix with Like a Dragon. SE competing with itself with DQXI, Octopath, and Star Ocean the Divine Force doing great. Soulslike JRPGs surpassing FF. Even small JRPGs being greenlit and pumped out every year.
All of these JRPGs have an effect on the overall budget for most JRPG buyers, and more people are passing on FF.