End-game in FFXIV:

THM(BLM)(Always stacked, doesn't need to be.)
WAR (Standard tank, even though PLD does just fine.)
ARC(BRD) (Always stacked, doesn't need to be.)
WHM (standard slot)

So it's not built around a set number of classes, it's built around what will be an auto-win button to the community and people denied this up and down for years with MMOs, Garuda was able to be burned down by BLMs and it pretty much threw their lie back in their face.

There's plenty of room for new classes, there always is, the problem is if the community will stop being retarded and actually utilize different strategies even if they don't work the first or 2nd time, everything has a flow you have to get into (though luckily the minority of the community actually try different things.)

Is there any room at all for a SMN in endgame
How does SMN work in XIV? We don't know. If you're basing it on how XI's work currently, plenty of room for it, especially if they keep the same general mechanic:

-Damage
-Crowd Control
-Minor Healing if needed
-Group Buffs
-Crowd Debuffs
-Aura Effects that also grants buffs

Plenty of room if people stop stacking classes. Yes you have 8 slots, no that doesn't mean 3 need to BLM.