When PS4 drops, the majority of gamers will still be on PS3. While a couple million will purchase release day consoles, the majority will not. That is a common misconception around here.
You also have to take into account that while some may trade in their PS3 to get PS4, the PS3 is still a very viable console being sold and produced for, with many great games still to come out for it, so it will remain active and many who purchase a new console will keep the PS3 right next to it as well.
People often say around here that new games will kill this off, and that is also a very close-minded idea. While some people may have such trouble multi-tasking that they are unable to play multiple games in the same week, many don't and will hop between COD and GTA or whatever games they are current playing.
Then you have to consider that Rockstar plans to support GTA:Online for a considerable amount of time based on all of their interview, so the 1-2 months of launch may pale in comparison to what thes future hold down the road, as is the same with most games of this type in the past.