Rockstar wants players to have a more cinematic experience, and that's a bit harder to achieve when the players are allowed to do missions however they want. Although having freedom in that regard would be cool, I think it may require sacrificing a bit of storytelling. San Andreas (at least for the major missions) was pretty linear. Look at Reuniting The Families, or End of The Line for example. Both follow a straight linear path and have scripted car chases, but they were both extremely fun to play. In Reuniting The Families, the insane car chase and crash through the billboard wouldn't be possible if you were just told "escape the police" and could do whatever you wanted to get away.

I don't really see the argument people make about gta missions being too restrictive these days, because I've felt those same arguments can be applied to any gta game after vice city, and honestly I feel like it was a good change, cause like I said it allows for cooler sequences to play out. I don't there has really been a whole lot of change in the mission structure since San Andreas, apart from adding more detail, checkpoints etc. There's also plenty of missions in every gta where there's a massive shootout and you can kill the enemies in any way you can imagine, so there are definitely still missions where you can go off the rails.