It's just a simple combination, and there's no reason they can't do all three:
We have the whole story mode thing - that's done
A lot of people would like to then start with each island being blank and you have all the constraints of the story mode, just without the narration and the mission structures
A lot of people then want to start each island being blank and also have unlimited funds and everything unlocked so they can just build what they want
In honesty I don't think it's overly difficult to do these two sandbox modes because they would essentially need to strip back what they've already built. All the islands must have a base blank template asset in the game so load that up, don't put the build boundaries as tight as they are, then also have a tick box next to each island that indicates whether you want unlimited mode or not
And as with these kind of things in gaming, if that doesn't appeal to you then you don't need to play it. We already have the game to a degree where you don't actually have to follow the story mode/missions etc to progress the game (turning those notifications off might be handy though) so you can choose to follow it or not
(NB: I say it's not overly difficult - I'm obviously not a game dev and so I'm just an entitled and assuming gamer, but I believe if Frontier have the skills to pull off these dinos that we're seeing then they can easily slap a sandbox mode in, probably just by using telekinesis at this point)
Really well said.