Is really about getting more games. The more you sale the more likely you will get a game made for your system. That is why it was so important for MS to come out early because you couldn't ignore the installed base. If you wanted to make money then you had to make game son the 360 while the PS3 or even Wii caught on. The Wii has caught on and passed it but even so not all third party have had as much success as on the 360.
If the 360 launched with he PS3 then almost all the games would have been made on the PS3 because of the PS2 having 100+ installed base it was common sense most would just migrate to the PS3.
In the end it doesn't matter this generation since the 360 has the installed base to continue to have games for a couple of more years and then like you said a new one will become available. The PS3 will also continue to do very well and will have plenty of game as well. Honestly all console owners should be happy this generation as all three systems are getting great support vs the previous generations you had a leader and the rest fighting for scrapes.











