Seperate consoles not selling much anymore would be incentive enough.
It's either going there or everything will eventually converge into "portable something" in one melting pot. Like "PC" or "portable machines".
Having a unified console would be fantastic though. The hardware could be designed out of a consortium and specs could be agreed on for at least 10 years. At that point, any hardware maker could actually make a machine, just like with DVDs, CDs, etc. The hardware market would actually have a huge boom. For devs and publishers that would be fantastic and a fiscal dream in the mean that they potential sales / userbase would suddenly triple (at least!!). Finally, for gamers it would also be a dream because the competition would rely entirely on the software side. This means general quality of games going up.