Personally I would be more interested in a 500 or 600mm prime telephoto than a 150-400 zoom... I use my Oly 300mm with the 1.4X teleconverter pretty much permanently attached, so I'm already at 420. I heard a rumor that when they have the big event coming up, they might also announce development plans for more lenses, maybe even a little "road map"...
As you probably know, the 150-400 is supposed to come with a built-in 1.25x switchable TC option, and Oly will also introduce a 2x TC (not sure if that will work with the 300mm prime as well). So it also comes with the promise of options for more reach. And yes, I believe they are supposed to announce two other longish lenses in development. I really prefer zooms myself because I need the flexibility to shift between different types and sizes of subjects.
I also agree with others that I view the greater depth of field offered by m4/3 lenses (versus lenses for full frame or A-PSC cameras) as an advantage, not a disadvantage, especially when it comes to shooting close subjects such as insects or plants!