We really need to hammer down a definition of "Isekai" for the conversation to go on. How it works is going to be pretty arbitrary.
Personally I lump Isekai and Planetary Romance together because I disapprove of dividing things by nationality and saying "Only X country can create X genre." It seems prejudicial to me and ignores how much interchange there is between fandoms of different nations. For this reason I don't generally divide them by whether they are Japanese or not.
By that standard
Sword Art Online is Isekai, as are
Flash Gordon,
Buck Rogers,
Alice in Wonderland,
The Wizard of Oz,
Magic Knight Rayearth, and
Inuyasha.
One can consider the "Changeling Fantasy" genre where a person discovers they were never human but rather a witch/alien/werewolf etc. and comes into their powers as a subgenre of another world, as the hero does indeed leave their old world behind and go to a new one, often hidden in the same world but dramatically changed by understanding that it has magic or aliens in it.
Harry Potter is probably the most successful current ChangelingFantasy. I would class that as a subgenre as Harry's Adventures are quite similar to a a japanese HikiNEET except he goes back to the ordinary world a couple months each summer.
Modern-style Isekai usually includes gamer elements and a HikiNEET protagonist in which case women were never included in those but I feel limited it to those elements is too narrow. If one expands it to include the likes of
Escaflowne,
Inuyasha, and
Fushigi Yuugi, which predate the gamer elements that were strongly popularized by Sword Art Online, one needs to also include the male-led Isekai of the time and those that preceded them such as the aforementioned
Isekai no Yūshi or beyond ancient
Urashima Tarō.
I do not like lumping "reverse Isekai" in with Isekai because it tends to swing a huge net across a swath of "Magical Girlfriend" romance pieces like
I Dream of Jeannie in the west and
Sally the Witch in anime (which was heavily inspired by
Bewitched, another magical girlfriend series in the west). This makes the separation less useful as you're pulling in a lot of stuff with an entirely different theme base.
Female characters have always been a major part of Planetary Romance and Isekai both.
Alice in Wonderland is a fine example of a bizarre world to be explored and Alice's irritation with it's perpetual insanity is lovingly crafted. But there is no period one can point to where only females were being Isekai'd, nor really males for that matter and the idea that Isekai is somehow "stolen" seems bothersome.