It's an interesting question. Creating a console is a massive undertaking though, I don't think that 'Sony's gone, let's do this!' would be a proper motivation for creating a console system, setting up connections with developers and manufacturers, especially if it's going to be on the same level as any of the current systems. Wii, 360 and PS3 cost immense amounts of money to develop and games for them cost as much. More importantly, it would take time, years of development for a new console from scratch. The next Xbox and Wii have most likely been in development since the release of the last generation.
If someone were to see Sony's leaving of the console market as an opening, we might not see another new console coming out for three or four years, probably behind the releases of a new Xbox or Wii console.
It might come down to just two consoles, one for 'serious' gamers, the other for 'casual' gamers. A more likely outcome might be that Wii would change it's focus from a more casual focus to a greater 'general gamer' focus, amplifying it's processor and graphics abilities to be able to compete with 360 in the AAA games market and being able to have major titles play on their system without a great deal of modification.
But I don't think someone would 'take over Sony's market share' simply because of the amount of time, they'd be working against Xbox and Nintendo, both of whom have more money, existing market share, and existing reputations with both consumers and game manufacturers, both of whom know what the consoles can do, and what they want from them.