I understand exactly how you feel in terms of how realistic it is. I questioned myself about the same thing in the writing. Entire armies never chase one person, especially throughout a populated city. But to be fair, let's consider how realistic it is for one man to go nuts and start killing everyone they see via a number of weapons stored in only his pocket, not weighing him down in the least. If this happened in real life, and this guy appeared to be an unstoppable force, then we may see the authorities turn to the armed forces. So in a way, it's something that could realistically be a solution, though of course such a situation in real life is very unlikely, but that's why it's a video game.
In regards to the return of the Rhino tank, I don't mean the return of other vehicles exploding on contact with a juggernaut of a vehicle. I mean not an APC, that just wasn't the same in TBoGT as the Rhink tank in the previous games. A new Rhino tank would not be indestructible, but it would of course be very heavily armored, and would mount heavily damaging artillery, but you wouldn't see vehicles exploding from a single shot taken at any point on the vehicle. That's where TBoGT's APC got it right, it would do heavy damage to other vehicles, but not exaggerating-ly annihilate them. However, the APC's artillery was not very aesthetically satisfying, some tweaking in the physics of it may help fix that. All in all, a new Rhino tank would have to reserve the threat it posed to anyone standing in its way as it has done previously in the series. With GTA IV's advanced vehicle damage system and physics engine, such a tank could quite aesthetically run right over another vehicle, crushing it relatively flat, rather than exploding on contact.
Realism in terms of gameplay (not in terms of creating a believably alive environment) is difficult to find an appropriate balance for in some video games that are not purely fictitious in their ideology. Hopefully Rockstar will be able to effectively and appropriately decide what would provoke further entertainment with their games and what would just be silly. Remember, while realism seems to be a must in future GTAs, the games also must be a lot of fun, which means potential sacrifice depending on how you view ideas as realistic.
Personally, my thoughts on the return of the army and tanks in GTA remain as they were, but I really appreciate the feedback you provided there so I could further specify my opinions and ideas on that topic.
By the way, I just updated the first post with some new stuff, check it out!