The term 'isekai' is actually poorly defined and subject to change. It is simply a way to group a selection of similar stories that the majority of people (in a community) have come to a general understanding of. With this way of defining it, different people will have a different opinion on how they should be grouped. They can argue for their opinion, but if the majority disagree then it is not labelled so. Of course the opinion of the majority is subject to change and why 'isekai' is poorly defined.
Here are some examples I have come across:
SAO being an 'isekai'. The main character is transported to a virtual world. The main arguments against SAO being labelled as such are that it doesn't fit with main characteristics of an 'isekai'. You can see how problematic this is.
Another example is Steins;gate. Yes the main character travels between different timelines. A different timeline means a different world has occured/ a parallel universe which is considered an 'isekai'. Some arguments against are that the main character isn't trapped in another world and again it doesn't fit with the standard model. If you define 'isekai' as transported to another world, Steins;gate is an isekai. Again it is problematic.
The last example is the weakest one, but you can see where it is argued from. That death note is an 'isekai'. If we can agree that the place where shinigami come from is a different world than the human world, then if it is argued that Ryuk coming to the human world is the most important thing (as it is the catalyst for everything to come) then death note could be considered an 'isekai'. Of course I also disagree with this one as the central thing to death note isn't about Ryuk coming to the human world, but Lights ideals and the cat and mouse between Light and L. You can still see where this is coming from though.
Examples based in real life:
Can dreams be considered another world? There are already stories based on the main character being in a coma and their mind being transported to another world, but can we consider everyday dreams to be another world? Am I an 'isekai' protagonist? Are you one?
What if we define our home to be a separate world to the outside. I can see a novel based on someone defining their home to be a different world from the outside mentally, but would the majority of people agree. The current answer would be no. If this novel was published now, I doubt it would be considered as 'isekai'. Maybe in the future when someone pushes the boundaries on what can be considered 'isekai'.
Crownfalls opinion:
Crownfall literally didn't bother arguing anything and just went reincarnation=isekai. Reincarnation doesn't necessarily mean isekai and isekai doesn't mean reincarnation has to happen. If we consider the state of being dead to be another plane of existence then it could be considered that the avatar has a short trip to another world each time they die, but this isn't the main focus of the story and isn't really defined. What if we considered the avatar defining each nation to be another world? Again it is not the main focus of the story and I doubt anybody would accept that. There is also the fact that the avatar has already died before and been in each nation before. As he has already been in these 'worlds' multiple times before, can we really consider them to be another world? I don't think so, they are already all the avatars worlds.
In conclusion, what the term 'isekai' means is honestly based on opinion (applies to all words honestly). The fact that the majority of people (in a community) have a similar understanding of what it means allows me to use the word 'isekai' rather than 'transported to another world' with a lot of footnotes tagged on.