I do not know what the hell is so important about full frame, but you going to have to wait a long long time for a full frame camera the size of the EP1. If it ever happens.
Narrow DOF and better high ISO because it can gather more light. A 43 sensor maker might be able to make "as good as" jpgs as a FF maker, but the if they put that into a larger sensor, they'd be better still! (of course, there are other trade-offs in lens length, but it would be nice to have both -- a 43 sensor AND a FF sensor both in small bodies).
Do you mean large FF lenses and a compact FF body as an accessory?
Would you like smooth CAF HD video with that?
In my opinion, DSLRs will survive in the FF format because of all the goodies a larger sensor provides. This also implies larger lenses hence larger bodies where a TTL optical finder will always have its place.
On the other hand the 4/3 DSLR might be at risk once CAF, EVF and bright prime lenses are available in the m4/3 format but this still a few years away. In the meanwhile, I am waiting for the E620 and ZD 50mm F2 I ordered last week because I cannot wait for this new system to be designed and my kids are growing up right now!
My thinking is that there is a place for tiny sensors (really deep DOF at wide apertures) for people who just want to point and shoot without focusing, and then a place for larger sensors for people who want more DOF and creative control. Also, physics and light being what they are, larger sensors can gather more light, so if you are using the same sensor technology and the same sensor processing, you can do better with a larger sensor. But, physics also demand larger/longer lenses on larger sensors for the same FOV.
So, there are places for all of these. Folks who think bridge cameras will kill everything are pretty naive. The 4/3 sensor sits in between and, IMO, is a compromise position. I would rather have a larger sensor in a body the size of the EP1, but as it's not yet available, the EP1 is a great camera, and I think many more like it will follow.
I think overall, this thread is a bit confused if it is talking the death of simply the mirror or the death of larger sensor formats, or the death of large cameras. If you could replace the mirror completely and lose the weight (and assuming cost is the same) then why wouldn't you. The question on the mirror side is, are EVFs "good enough" to replace mirrors. The answer is -- not quite yet (at least not as implemented) and I doubt within the year.
At the same time, will the 43 sensor replace or obsolete the APS-C or FF format sensors? No way, but could a 43-based system become more popular in the short run? It's possible, but that will just accelerate the release of smaller, full-featured APS-C and FF cameras (which is a good thing, IMO).
43 is introduce camera sizes that people want, but whether it will establish a true place in photography long term, or is just a passing format until larger sensors shrink in camera size, is a question I don't think I could answer, except to say that in years past, there were also Kodak instamatics, true DSLRs, consumer DSLRs and rangfinder-types (including PENs). What's to say that's not how it will shake out in digital? Each size/sensor combo has a unique set of strengths and weaknesses going for it.
