I don't see online being that big of a deal in collapsing systems down to one sole survivor. Unlike computers in the olde times where people on different systems really needed to share files and send information to one another, there was a very strong demand for compatibility (remember the phrase: IBM compatible?) once office and home use started to get going. The game console generates no such necessity, it's merely an entertainment medium. As long as there are enough people in the online world for each system then they will be able to continue as separate communities indefinitely.
In any case there is no reason why 2 distinct systems can't interact when desirable (e.g. with multiplat games), while maintaining separation for exclusive content. Different mobile phone systems (CDMA vs GSM) talk to one another, there is no reason for a multiplat online game can't be played with some players on PS3, others on X360 and others on the Wii, other than the unwillingness of the companies to make it happen, and the lack of demand from the consumers for it to happen.
“The fundamental cause of the trouble is that in the modern world the stupid are cocksure while the intelligent are full of doubt.” - Bertrand Russell
"When the power of love overcomes the love of power, the world will know peace."
Jimi Hendrix
