If it’s stated it’s limited time, then correct - you should push for the item before the time is gone (if it’s something you care about). If it’s also promised they’re “gone forever”, then you should also consider that, when it comes to applying effort to the given activity.
A good example is the new 3k mount achievement for Mythic+. I know darn well it’s a FOMO item, because that’s how the system promotes it. I didn’t participate in m+ this season, for various reasons; I’m not being a FOMO andy about it, and no I don’t think it should ever be released again.
Legendary handouts (which is what they have been since MoP) are not comparable to limited time achievements. Not even in the same ball park!