Spoiler ShowWell Destiny Islands was essentially the final boss area of 0.2, so I mean it served it's purpose in that game. In BBS, yes they could have very well easily made Destiny Islands playable, heck people have hacked BBS and had the trio running around Destiny Islands as if it were playable.
But the difference between that situation and the what we see being done with Radiant Garden in KH3 comes to down to WHY they should be playable. Destiny Islands served it's purpose in BBS in cutscene form, there was no need for it to be playable. But Radiant Garden is different. This game, Kingdom Hearts III, is the finale of the Xehanort Saga. Radiant Garden almost more than any other world in the series as been the center of the Xehanort Saga. Think about all of the important defining events of the saga that have occurred in this world:
1. Kairi gained the ability to one day wield a Keyblade here.
2. Terranort was found here and became Ansem the Wise's apprentice.
3. Through the experiments that Xehanort and the other apprentices conducting in this world, the Heartless began their invasion of the Realm of Light here.
4. This is the home world of the first 8 members of Organization XIII, this is pretty much where Organization XIII began.
5. This is the home world of Kairi.
6. Leon and the gang, Sora's first true friends outside of Destiny Islands and the ones who first explained to him and by extension the player the concepts behind the Keyblade and the Heartless, reside here and their lives have been negatively affected by Xehanort as much as anyone else.
7. This world is stated to be the capital of the realm of light by Xehanort himself and it was so important that Maleficent chose it to be her home base.
8. Sora became a Heartless here, which means this world is where Roxas and Namine were created.
9. This is the world where Aqua fell into the Realm of Darkness.
And there are probably some points I'm forgetting. But my point is, that from a narrative standpoint, Radiant Garden has been such a central focus of this entire saga it just feels wrong to me to not include it as a playable world in the finale of the very saga it has been such a massive part of it. And I don't understand how people, especially the ones that have been playing the games since the first one came out, can really sit here and say they are okay with Radiant Garden being included. It's just something that doesn't feel right to me.
And the fact that that Square would spend time to make all of the resources for Radiant Garden and then have it just be used for cutscenes, is like them teasing us, and it's frankly insulting, because they have to know how much fans wanted this world to be in the game. I'm sorry, but I don't care what excuse people come up with to try and defend Nomura's decision here, I just can't accept any defense of this decision as being valid. That's my honest feelings on this, and baring by some miracle the game comes out and the world is actually playable, I'm not changing my opinion on this issue because I feel that strongly about it.
Finally I will say that Destiny Islands should have been playable too considering it is the home world of our main hero and main villain.
...Just so you know, Nomura confirmed that quite a lot of the backgrounds in many of the cutscenes are not actually in engine, they are pre-rendered. It is entirely possible that Radiant Garden is just a pre-rendered background for cutscenes.