Great perspective. It's plausible that Rockstar ended up losing focus of what we intended to purchase Grand Theft Auto V for. However, I wouldn't say Grand Theft Auto Online is what ruins GTA. I'd pinpoint exactly what the problem might be -- the developers possibly focusing much more on the online feature as opposed to the single-player experience. GTA Online gave GTA more appeal, possibly much more public appeal than ever. Personally, I think it's great that Rockstar decided to implement this amazing online game. I've never been so hooked with online console gaming, and there's no better way to do it than to do it with your favorite game series.
Additionally, I think this online feature gives players a great alternative to free roaming in single-player mode. Why play minigames and roam around Los Santos alone when you could do it with others? I know that GTA IV's online feature never had much appeal to me, so I stuck with single-player story mode even after I finished the story. I roamed around Liberty City and Alderney City, but that gets boring quickly, especially since you're already roaming around the two cities as part of the storyline. There's not much to explore anymore. In my opinion, the online mode is making the game worth the purchase. Players are getting much more than they paid for, especially with new downloadable content continuously being released. What you get is basically a whole new game free of charge. I honestly didn't expect Grand Theft Auto Online to be so great. I thought it would be similar to the online mode of Grand Theft Auto IV.
With that being said, I hope Rockstar doesn't lose sight of what made this series so remarkable in the first place. I like playing games for the storylines, and to be honest, Grand Theft Auto V seemed hallow in terms of the story.