...maybe I'm a bit cynical - and I actually havent played FFXI, so I'm not sure what Hamlet Defense actually was like in that game (even though I heared great things), but... we got Eureka which was supposed to be "FFXI-like" content.
And it didnt/doesnt work - for several reasons. Some of them are actually variations of "this content is just badly designed", BUT lets please not forget that it "worked" in FFXI because that game worked differently in regards to the battle system and gear progression and so on.

I might not know enough about Hamlet to actually make a call on wether or not that could work in FFXIV, but at this point I'd very much like the devs to developed content for this game, playing to the strengths of it. After Eureka I'm very sceptical about FFXI-insipired content - something that was great 10 years ago, might not be so great anymore now.

So, no, I dont feel they've missed a chance there - maybe they've missed a chance to make the maps feel more "alive" or like a battlefield, but I dont believe that asking for FFXI 2.0 is the answer here. They need to come up with ideas that actually fit this game - and I cant tell wether or not a tweaked Hamlet could do that, but I'm pretty sure that open world content that needs a lot of people participating just doesnt work in FFXIV. Sitting around, waiting for something to happen might have been great in FFXI, but not here.