It's been a dying series for a long time but not for any of the reasons you outlined.
Alienating fans and limiting growth by not putting the game on Xbox? There have been several games on Xbox and they sold next to nothing compared to the other platforms the game was offered on. Do you seriously think that SQEX aren't looking at historical sales to determine how they are spending $100M+?
Putting the best team on remakes? I wouldn't say No please no-mura and co are the best that SQEX have to offer. And new IPs are even more of a financial risk to a company in SQEX's position.
And then FFXVI… Which was a good but not great but to say it had "unforgivable shortcomings" is not only off-base but it completely unwinds your argument about new IPs. You think they should develop a brand new title with mechanics that differ from traditional SQEX JRPGs but scoff at new mechanics. Maybe port begging is really the game you'd like to continue playing from SQEX.