Lucas, remember that there was also 7 GTA's before GTA4, although not all were for xbox I don't think.

GTA1
GTA2
GTA3 - Liberty City
GTA Vice city
GTA San andreas
GTA Liberty city stories
GTA Vice city stories

And now we have GTA4

GTA games will continue, there is no need for them to stop, it has to be one of the most wanted games everytime they come out.