https://ffxiclopedia.fandom.com/wiki/Spriggan_(MON)
Now, us two or three ppl who play MON may have needed these to complete our collection. However, there is something bigger at play here........(Just stay with me, I know ya'll hate my threads).
If they can add and remove Spriggans, then they know the system well enough to add and remove monsters. This brings me to my point and suggestion for a Monstrosity update.
I would wager Monstrosity was on par with besieged in terms of popularity among players. Besieged however did not have a 50 page thread after it stopped receiving updates. My point is, Monstrosity deserves updates just as much as besieged.
It appears to be a time thing with the team, they do not want to utilize time on Monstrosity. Time is indeed money. Clearly not a tech issue if they are adding and removing Mons.
Another issue is how many players are playing this system now? Well, not many perhaps, because it is a system reliant on unlocking monsters and there are no new ones to unlock.
If they do add new species, give us the Tonberry Master from 7 with the stars over his head that may have inspired master level player stars (Little detail I noticed for some time).