How would you propose doing that when customers aren't of a single mind about what they want?
There's a point where most people get bored with something, no matter how good it is, simply because they've done it too much. They need a break to try new things. Sometimes they'll return to the old, sometimes they won't.
Investing money trying to keep long time customers ends up a waste of money when they quit anyways. That's why so many business have special introductory offers for new customers but won't extend any special offers to their existing customers. Either the existing customer is still satisfied with what they're getting and will stick around, or they aren't and will leave.
How many of us are still actively playing one or more of the online game we were playing a decade ago and enjoyed? Of those games we're not still playing, why did we quit?
FFXIV will not be any different in the long run. There will come a point where we lose interest for whatever reason and move on. We owe no loyalty to a game as it is a business arrangement. We pay for access to the service because we're happy with what we're getting, or we don't pay for access to the service because it no longer suits our wants and needs. It's rare for the changes to a game over time to stay in sync with the changes to what a player is looking for. Eventually they're going to diverge and part ways.
The best thing to do is move on instead of trying to deny and fight the divergence. Sticking around won't accomplish anything. The developers are going to make the game they want to make, not the game that players think they should make.
But it never did require any "mental agility". All it required was experience. Once I figured out the differences between the WoW content style and the FFXIV content style, the casual content here became easy since a lot of my WoW experience could translate.
You're making the same mistake others have made. The new content additions are not easier than the older ones (not by a long shot) though design has changed. The difference is your mastery of the game. You got good. The challenge is no longer there.
If the game no longer offers the challenge you want because you've mastered it, then you're making the right decision to leave so you can find a game that will offer you new challenges.
But honestly, leave the petty insults like you used in your initial post out of your feedback. You're only tearing yourself down by using them because now you've made yourself look small and petty instead of looking like someone making a considered decision about what is in their best interests.