@mahtan Um...What?! Ignoring Planet of the Apes...what exactly makes 'space story' different from a 'medieval fantasy story'?!
It's simple. Either 'isekai' is every time a story talks about humans out of Earth (or any other species out of their own native planet and/or dimension), or it's only when a person leaves their own dimension into another one.
You seem to be supporting Wataterp's definition, and yet by his definition Star * are very much 'isekai', because they take humans out of Earth and into many other planets. It's even more of an 'isekai', since there are many OTHER races that leave their own home planets. Some of them very much unwillingly. You cannot just say that the 'home' in this story is Earth and in that story is entire universe (if it was just 'space', Star Wars would still be 'isekai', as planets are NOT part of space, they are IN space, like a rock in water is not a part of that water).
The fact that the genre of 'isekai' was not yet discerned as its own thing back when there were stories that very much fit the bill is irrelevant, and I don't have any idea what you try to achieve by even bringing that up. The term obviously was made
after there were stories that fit it. More, after
enough stories that fit it were made to give value to elevating it to its own genre. After all, unlike in video games where game genres affect mechanics which are more unique, stories involve merely fiction, and by definition, each story is distinct. Making a 'genre' for two or three stories would make millions of genres by now.
As for Sword Art Online and similar, you said it yourself. Some consider them 'isekai', some do not. But even those that do have more of an argument than in here. The consciousness of said characters is 'locked' in the game, and possibly tied to their life. .Hack (the game) had people trapped in the 'game', as well, in the truer sense of the word. I imagine this theme was also part of the other stories, but I've not read/watched anything other than the first, I believe, game. Even if you ignore THAT, there is still the concept of it being a foreign world of sorts. Just like we say 'real world' and 'digital world', getting 'into' the digital world would still be different than going to another place that you could walk to (if there was a super-walkway between the two planets and you'd have infinite lifespan). A different world governed by different (artificial) laws.
In Sentouin, the character simply teleports to another planet. It's no different from warp-drives/hyper-drives/wormholes/what-have-you in 'space stories'. He can move freely between them at this point even (though he does not want to).