Generally speaking, games that deliver incremental upgrades to past experiences don't attain the same sales levels as their predecessors. While GTA games may seem to defy this in that each installment has sold more than the previous, you have to consider as well that like most games, their lasting appeal is not that great. A week or two after a new GTA comes out, the past iterations drop off in sales dramatically as they become seen as inferior to the latest one by prospective buyers. And of course, thanks to gamer drift, some people end up dropping the series after a given iteration.
These two factors combined ensure that games like GTA don't actually sell much more than their predecessors even in a best-case scenario. In terms of the userbase at the game's launch versus continued sales after the newest iteration drops the game from its perch as "GTA of choice", each GTA has managed to sell to progressively smaller amounts of the installed userbase at the time of their GTA-series dominance.
To illustrate:
GTA3 ~ 11.60m on PS2
Sales Dominance: 10/23/01 to 10/28/02
Userbase Peak: 33,882,237 (~34% of userbase)
GTA:VC ~ 14.20m on PS2
Sales Dominance: 10/28/02 to 10/26/04
Userbase Peak: 70,961,023 (~20% of userbase)
GTA:SA ~ 15.45m on PS2
Sales Dominance: 10/26/04 to 04/29/08
Userbase Peak: 117,345,554 (~13% of userbase)
This is not 100% accurate, of course, but as games with front-loaded sales, it holds a certain degree of accuracy.
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