The only thing you will ever hear before player numbers plummet is comments like these here, saying they would like the game to be more challenging in at least a few areas, and saying they believe having 95% of the content be designed with the bottom 15% of the playerbase in mind is not accessibility, its boring.
There is no other big warning sign that your game got boring or that more people are on the verge of losing interest than that - and there have been many games, some of them well-established, that have had droves of players leave at some point. Sure, you can get some back, and get some new ones by then doing a drastic overhaul (ARR itself being such a thing), but it would have been better to not have to do that in the first place.

From a game design perspective, complaints about "boring", "autowin" and "too easy" are significantly more dangerous than complaints about something being too hard IN THE FIRST WEEK OF A MAJOR PATCH.

Players will adapt. The most successful games today, with 50-100x more players than FFXIV, are not easy and do not offer the option of turning down difficulty from 3 buttons needed to 2 buttons needed. They are competitive PvP games. In the PvE sector, we have games like Left 4 Dead 1+2, Vermintide 1+2, games that again are pretty rough and scale up to high difficulty levels.

The problem is that you need a ramp up, not a sudden jump. We are not incompetent. Only few people are genuinely unable to defeat Savage content - its just that the gap from what they have been taught to be enough to complete something, and what is now asked of them, is too big, and thus the effort to learn feels not in relation to the gain.

Almost all the new content in 5.1 caters to players who want more, for less effort. Who want to not bother really playing their class, but just kinda "be around" for the activity. Who want to be max level, BiS faster with less effort than before. Its a big big nerf to playtime of the game in general, and it caters solely to one type of player (Hades EX being the sole exception).
The last time this was done, in SB, we saw a massive dive in active players a few weeks later.

Solutions to this are not "make everything hard". There is plenty enough content for players who do not want to play their class beyond 50% efficiency, and has always been. The entire MSQ, NG+, almost every single side activity, is suited to them (and a snoozefest, gameplay-wise, to everyone else). What we lack is good gateway content to lead a player from the end of the MSQ, through the max-level dungeons, through the normal raids, the ex primals, to the savage raids, and then fill out that progression with more content.
I understand that not everybody might have an easy time with the mechanics in E2S, while maintaining DPS. That is okay. The fight asks for a very specific set of skills, and doesnt require much else. But its NOT good when that is the ONLY fight you have at that point of progression.