There's nothing wrong with Tiamat not spawning for 3~5 days after he is killed.
Yes, there is. It creates the whole 200 people camping, sitting around doing nothing during spawn windows game. What was fun about that? Mobs don't have to pop once a year for their stuff to be rare/desireable, or to be challenging.

Open world HNMs, in the form they were in in FFXI, suck. period. There's a reason WoW only had like 2 of them for several years. Nobody wanted it, and almost nobody even cared to try to fight them. FFXI-style open world HNMs are open to abuses that force pops and instanced NMs aren't. They should only put things like this in the game if and when they can resolve those problems and abuses.

its as if some of you can't see past FFXI when it comes to this game.
No. I'm seeing past FFXI right now. That's why I make the statement about issues needing to be addressed. If they rip HNMs from FFXI like they've ripped a lot of other things, it will suck. If they do it their own way and solve the problems that FFXI HNMs had, then I'm all for it.