The cops are going to follow you inside anyway. Going inside a building won't stop the "telepathic police department". The most useful this strategy could be would be the Assassin's Creed style where the character quickly sprints through the room and jumps out the window somehow losing the pursuers.
Going indoors is a death sentence in GTAV because the interiors, unlike actual buildings, have only one entrance.
How would it take "way way way too many resources" when the entire world--everything but the interiors is already completed? Did you not get a main point of my post--that if resources are not spent creating an entirely new world from scratch, then this would be possible? Look, I understand that you might not agree on the benefits of doing this, but you can't claim that Rockstar doesn't have the resources. They could do this and purchase a small African nation if they so chose.
Yeah they could do this. It'd also take many years since unless they want their game to become bland after an hour, they'd need to split the development team up for missions and writing. They'd use up resources making interiors so people can reenact Assassin's Creed chases when being pursued by police rather than just stealing a car. But like what was said, V's map was made by 1,000 people. Either 1,000 people work on interiors and everything else suffers or 500 people work on each and the game is released in the year 2031.
Buildings that the mission is going to take place in yes. There's no point making the ENTIRE city have interiors because some missions take place within interiors.
You want more gameplay opportunities and story. Expanding the world lays the framework for providing this. Gameplay opportunities created to take advantage of fully realized, inhabited buildings is, again, my point.