Portal fantasies aren't exactly new.

The Chronicles of Narnia, Alice in Wonderland and The Wizard of Oz are classics that aren't generally considered lazy.

Isekai tend towards the lazy, self-insert escapist portion of portal fantasy, sure. Most don't have great writing. But keep Sturgeons law in mind - every genre has a few gems in a sea of turds.

There was an original framing story about an Anglo Saxon sailor who finds the straight road and ends up in Valinor.

Tolkien kinda has something like that in story with the shire being fairly disconnected from the outside world. It still allows for some exposition, but also fits much better into the themes and flow of the story.

That's true, other writers use different methods to explain things to the audience, therefore this entire subgenre is just lazy for not doing it the same way