The problem is not extremely short lived content.

The problem is badly design content. People isn`t leaving content to die, content arrives dead, we basically try to CPR it for 2 week to a month and then we go "nope... this isn`t working". Diadem, Hunts, Goldsaucer, now Dead Palace... All of it could be still alive and kicking. At the begining i was also thinking "they are too short", but i have found that in reality "they are too bad/grindy/unrewarding/troublesome".

Diadem was troublesome and unrewarding most of the time, RNG based rewards, came to us dead.
Hunts was grindy, badly designed from the board, in one patch was moved from "the side activity" to "catch up activity" and never really became relevant again.
Gold Saucer... Chocobo races are a grind. LoV is a mechanical/control clusterf**k. Triple Triad is ok, but tournements are rigged in most servers and it basically become another "fill this collecting log" activity.
And now palace of the dead is just RNG progression of doing the same over and over endlessly, probably floors from 51 upwards will be releasing rewards to "keep up" with later patches... Again "catching content".

Instead of having value on their own, and being supported for longer, all these just have value because there are goals outside them that are important.