I'd rather have more meaningful content than endless repetition of identical side ops in an identical context. I also disagree sandbox is incredible, as it feels completely dead outside of the guard posts and bases. There's nothing explore, it doesn't feel alive and traveling is a bitch. As I said several times, the gameplay itself is excellent and I agree with you that it feels much more like a polished current gen title than most other games, but that didn't stop me from getting bored somewhere between the 50 and 100 hour mark. I pushed on far beyond 100 hours and completed all main story missions (including 45 and the elusive 46) and several of the higher difficulty missions, plus around 90% of the 157 side ops, but so much of it felt like a chore.
To me thats the difference between a sandbox, which is what I consider MGS to have, and a true open world in the GTA style. The open world has lots more going on, where the sandbox gives you the scope to approach more contained tasks in different ways etc.
EDIT - Just read this post on a forum - sums it up well for me:
Does the game have an empty open world? Yeah, sort of. But the world is intended to be a sandbox playground for you to execute missions around outposts (which are very well designed). It is not Witcher 3 or GTAV or RDR. There is no intention for getting the player to invest in the open world other than approaching defined missions with total freedom.