As a member of the Ultima Underworld team, I'm looking forward to seeing what Jimmy has to say about it. It's been really fun to watch him cover the stories behind the games of my youth.
As far as the Elder Scrolls go, Daggerfall was a more detailed game than either Oblivion or Skyrim.
The complexity split happened when every game started needing full voice acting to be marketable. Before, text was perfectly sufficient, and a lot more flexible.
When text-to-speech algorithms approach real voice acting, game world depth will start to notch back up.
I love keyword based dialog.
It's the feature I miss the most in modern RPGs, specially in the "total freedom, sandboxy" subgenre.
I remember it in Morrowind. It was daunting at first, coming from more traditional RPGs. But then it clicked. The ability to ask anyone anything is, may I say it, realistic.
As @mratzloff said, voice acting killed this. It is also my opinion that voice acting impoverished dialog in RPGs greatly.
it comes in a preconfigured dosbox
Ultima is to this day the single most impactful RPG ever made in terms of its influence on future games, it seems nothing new has been done since, almost.
Ultima 6's keyword-based dialogue is in my opinion superior to today's dialogue trees, some of the keywords would be highlighted in the conversation, but some you would have to come up with on your own from your adventures.
Ultima 7 has a much more detailed world than even the modern Elder Scrolls games, its breathtaking how full that world is; Elder Scrolls is still playing catch up almost 30 years later, insane!
Ultima Underworld was also tremendously groundbreaking, it's real, texture-mapped 3D (in 1992!) and does all the things you'd expect from a modern RPG, which is why I am excited that "the band is back together", so to speak, making Underworld Ascendant (no Ultima branding, Underworld was originally not an Ultima game, but was rebranded and reworked for marketing reasons)
I could ramble for hours about these old games. But I'll stop here, go and play, people. :)