Anyone can start reading ASM with
any issue. It's no harder to get the gist of what's happening now jumping into the book cold than it was for people who jumped onto the book ten, twenty or thirty years ago. And the disadvantages of a long running narrative are shared by every single other Marvel and DC superhero title so that's not an ASM thing, it's a comic book thing.
It's not bad to acknowledge that starting to read a comic that's been in publication for years can be daunting for newcomers - that's a challenge that the comic industry is always aware of - but it's wrong to make it seem like ASM is somehow any more difficult to enter into as a new reader than any other long-running book.