Take into account that GTA Vice City had 67 Main Story Missions (inc. Assets) and San Andreas had around 104 (inc. Assets)
70 Missions isn't too bad based on how well previous GTA Titles have done, these mission stats don't include heists by the way, so take into account all the planning and preperation side missions that need to be done to organise heists, these are just CORE missions that each character has.
Who cares how many missions it has, I certainly won't be rushing the storyline, they world is set up to explore, and that's exactly what I'm going to do.
If single-player starts to get boring I'll just head over to GTAO once October 1st Hits, 700+ missions on there...Sorted.
Stop panicking saying the game won't last long or it'll get boring.
£170,000,000 is a lot of cash to splash on a game that Rockstar hadn't given a core storyline too, so maybe they have cut the quantity of missions down, but I'm almost certain that the quality of the missions will have you hooked for hours.
100+ hours of Singleplayer gameplay, most likely 150+ hours to get 100% completion on the game.