Sanest take
So I noticed that some fellows are talking as if the "isekai" genre was a fad, some shiny new gimmick that is near the end of its ability to keep attention. [...] But I would like to take a moment so we can remember that "I find myself suddenly in a new and strange and magical world!" is not a new idea.
If you equate isekai to portal fantasy, there's no point in using the word "isekai", and yet people still do
Also, you should worry first about what your fellows refer by "isekai" when they make their statement. Language, words, and categories have no real basis except fulfilling sortal function for convenience of communication.
You interpret "Isekai is a fad" as "portal fantasy is a fad", but what they could mean is "stories like mushoku tensei, re:zero, slime tensei, konosuba are a fad". As you can see, these two statements are radically different.
You could rename dogs as "cat" on the basis that they both have four paws, a tail, and a cute mug, and no one would care. This would only hurt yourself and your interaction with society by creating misunderstanding. Like someone would say "cat are easy to take care of" and you would retort "cats (dogs) need a lot of attention and care!".
There's no real authority when it comes to communication. Even dictionnaries or encyclopedia can only attempt to vaguely point out characteristics, but there will always be a tricky bastard who enjoy finding loophole in definition to shove something absurd into it.
If you offer a soup to your friend and it turns out to be a bowl of cereal, their reaction won't be "haha, that's right, cereals in milk fits the definition of soup". No, it will be "this bastard is a sociopath, and I really should reconsider our friendship". In such a case, definitions should be improved to describe the usage of the word soup more accurately in society, instead of necessarily just accepting cereal bowl as a soup.
Sorting reality comes first and definition comes after. Corty refers to
@Corty. You can have fun defining Corty as a person who post AI-generated cute girls and no one would care. Then according to that logic defined, you could call
@RepresentingEnvy Corty and you'll be internally consistent. The only issue is that other people will look at you strangely like you're an autistic retard because everyone else agree in another usage of Corty. In this context, the genre "isekai" might poses a similar problem that if you reduce it to a single characteristic and shove all kind of absurd titles in that category, everyone will look at you funny.
Even the word sandwich can be up to debate:
In conclusion, yeah portal fantasy isn't a fad alright, but maybe you're pushing the "isekai" genre way beyond its common usage from readers.
If you tell me that Dante's Inferno is an isekai and there's no filler chapter praising the japanese cuisine and the nostalgia of rice and soy sauce, you'd leave me with no choice but to question your trustworthiness.