Granted, I do agree with you in that 'their usual MO' is not the strongest argument. In fact, I'd even say that it's a terrible reason that Bethesda could cite for not announcing at E3. However, I honestly can't think of any big games that have ever been announced at E3. Showing off the progress of a huge game, previously announced - not that's a different matter entirely. Sure, there have been games announced at E3, but none I can think of that amounted to much. :shrug:
Plenty of big games have been announced! Just last year there was Halo: Reach, the latest Final Fantasy game, and more I can't remember.
As you said, both of them, also Mario Galaxy 2 and the next Zelda (although no information was give, just the sole concept art). There were a few more that looked cool but not in my particular series' intrest.
as for dev time, open-world games have to take longer. Think, a linear or mission-based game only needs to be debugged along certain missions. A sandbox game needs to be debugged for even longer, and that is JUST post-production. Then there's story, a game with so many factions and side missions along with the main quest needs a ton of writing, acting, coding, etc. Far more than any other genre. Of course, then there's pre-production. If what Todd said is true, and they want to be more original and bring back the cultural feel of Morrowind then even pre-production would take longer because they need to brainstorm and do it hard.
It's gonna take a long time, but I believe it'll be worth the wait.