I really doubt you'll see anyone else being stupid enough to try and get into the console business right now. As it stands, the big three just have too much market share, fanbase, and economic power in the market. The only reason Microsoft was even able to make a good solid go of it is that they're the richest company in the world. It isn't cheap to develop a new console, or set up manufacturing for it. I think most investors would be smart enough to realize that there's no room for a fourth console to come out of nowhere and compete in the market right now.

As for whether or not consoles will be replaced by something like handhelds; I doubt it. I personally don't like playing handhelds. I don't travel enough for it to be worth having for long trips, and when I do travel, I'm usually doing the driving so they're useless to me, and if I'm at home, I'd rather play on my 27" TV with surround sound. Handhelds are huge business, but a replacement for consoles they are not.