I think the issue is comparing a floor with a ceiling. Fire Emblem and Xenoblade becoming more popular in the west in comparison with their own previous entries shows growth for those series. Yet in an absolute sense their sales are still far below what FF typically sells.
FF still outsells them, but is dropping in popularity such that it is falling out of the tier it needs to be to be considered a solid success.
A Persona game selling 5 million can be considered amazing for the series. A FF game selling 5 million can be considered a financial loss for the company.
That's the difference. So if we want to really talk about how the ceiling of JRPGs are not declining, we will need to find JRPGs that continue to sell at the level of success we have seen them hit previously. Where are the 8-10 million sellers? Are there any?
The ceiling might be dropping, sure.
But I think the pool of games that ever sold that many in the space were solely of the FF brand name. Most JRPGs never sold that much and had to keep a tighter budget. Perhaps FF should be doing the same, but that's beside the point.
Basically, it is absolutely unique to the Final Fantasy IP and it has to do with how badly they have tarnished the brand name with 20 years of middling fan reception while constantly throwing the baby out with the bath water on the design side. FF7 remake is collateral, it really has to do with the overall brand decline.
I think, on a personal note, that PC sales will make this a near non issue and that they will ultimately close the gap between remake and rebirth (though a sequel will never ever sell the same exact volume, regardless of quality) but I do think they need to reevaluate the brand. If that means tightening up the budgets, then they should go for it.