It's late and I'm tired and wanted to save this for tomorrow but I'm busy then so may as well have a good crack at it now
But the thing is... I don't consider Radiant Garden a "hubworld" like Traverse Town was. I see it as an original world which -- of course -- had main story elements unfold there, just like Twilight Town was in KH2. I think the only true "hubworld" in KH was Traverse Town in KH1 because in that game there were things you can only do in Traverse Town.
You basically answer your own question as to why KH1 was so different with regards to synthesis and shops later on, but I think it's a little reaching to say that a hub world is purely a place where you're forced to go to do *things*. I don't think that's true at all. Of course I do think it should have that something extra that sets it apart, normally it's Winnie the Pooh, and some extra shops, which is more than enough for me.
More importantly though a hubworld to me is something that "feels" like home. Leon, Yuffie, Cid and Aerith feel like Sora's friends and mentors, it feels right to check in on them periodically, because that's what someone like Sora would do. It also doesn't hurt the sheer amount of side plot stuff that RG has going on.
More importantly, the characters themselves provide some of the best and most interesting interactions with Sora which just further cements the feeling of belonging that a hubworld is suppose to invoke.
I think the question is do we need a hubworld anymore in the KH series? A world that we have to keep going back to in order to advance the plot?
Yes, yes, a hundred thousand times YES. The answer should never be a No. Just because recent KH games have moved away from this does not make it okay. Its telling that DDD and KH3 are as polarising as they are. Of course it would be remiss of me to say that's down to the lack of a hub world, that's not the root cause at all, but they are still problems and they correlate with my point.
A world that is deeply connected to the lore and to the characters, whether historically or over the course of the game like Traverse Town originally was, a world where you can go to have a breather, relax and enjoy the views, a world that consistently matters, is always highly preferable to have.
What's the difference between periodic visits to a hubworld that expand the world every visit and constant romps through disney worlds? One of these gives me a lot more enjoyment than the other personally. It's not like I can traipse through radiant garden in any other game which isn't the case for a lot of disney films, I can only have that experience through KH. It's disheartening to see fans so willing to throw out almost everything that makes KH for....? More Disney worlds or something? I'm not entirely sure what a suitable replacement would be.
but with the Gummiphone and the impatience of players/convenience demanded by players nowadays, I don't think a hubworld where you have to visit in order to do certain things will be met with a positive reaction.
Again, what's the difference between going to a hubworld to unlock certain events and going through disney worlds to get to the end of the game? This argument in particular makes very little sense to me, it's not like the RG episodes in KH2 were strictly formulaic. And as for your point about gummiphones and new players, well frankly it's not an issue. I'd say if you gave KH fans the opportunity to visit RG themselves or to just have Ienzo espouse exposition from a facetime call, I think it's safe to say that the former would be the clear winner.
Gummiphone interactions are boring and ruin the fact that as a person playing a game, you should be experiencing these things first hand, not through a screen in a screen. Also I'm not paying £60 for convenience. KH games have almost always been geared towards fans, with the exception of KH3 and its weird half and half nature. I'd go as far as to say when it comes to these things appearing in KH, F*ck the casuals, they don't appreciate what makes this series so unique. And anyway, there's no need to gear KH towards players who won't give it the time of day until it changes itself completely to fit their image of a good game