The streaming model isn't simple though. You need to decide which assets to load, when, and when to discard them. You also have to consider how your level is designed, i.e. if you've got three tiles, and a player travels from tile a to b, and then b to c. When he moves from b to c, you can remove a from memory, but what if zone b + c is too big to fit, whereas a to b is okay.
A really interesting presentation was given at GDC this year from Insomniac games on streaming http://s3.crashworks.org/gdc15/ElanRuskin_SunsetOverdrive_St...
Also, when you get close there many ways to hide the loading going on so it seems cleaner. Like a player going from planet A to planet B though a more limited space ship. On arrival they see a larger but still limited space port, giving the game time to load the new planet. Or even just boosting the glare when someone steps outside.
However, IMO these things can easily be over done.
PS: Skyrim is an interesting case where there is a player made patch to make city's open world where the original game has a loading screen. http://www.nexusmods.com/skyrim/mods/8058