Oh bullshit! You may think its the best thing since sliced bread but for every guy like you that will beat the thing there are a good 100 that will get bored halfway through and end up dropping the game, never to return. This has been documented and just because you are a fanboy of the series will NOT change those figures. For the record i have GTA III, GTA:VC, and GTA:SA, know how many of those I finished? NONE, that is how many, as it never fails but the GTA games always end up feeling like a slog once you get a little passed the halfway point. The second a game starts feeling like I'm punching a clock instead of playing a game? I'm out of there and the figures show I'm far from being alone in that sentiment.
For the record the only sandbox games i had enough of a fun time playing the missions to complete were Just Cause II and Saints Row games, JCII because they kept ramping up the "cool guys don't look at explosions" badass-o-meter and with the saints games the missions kept getting more and more batshit so I stuck it out just to see how crazy they got by the end. With the GTA games it always felt like you had seen all the best stuff by the halfway point and they started feeling repetitive so i bailed. Just because this guy doesn't feel its some epic adventure doesn't change the fact that the majority probably won't see the end of GTA V anymore than I saw the end of GTA III-GTA:SA.