SAO didn't broke a thing. Isekai was popularized on Japan due to Shosetsuka ni Narou, which is where the boom of web novels began at the end of the 2000s. It's also the reason for why publishers like Kadokawa, Shueisha, Shogakukan, Kodansha, Micro Magazine, SB Creative and many others began to pick up popular works from Narou and other websites like Arkadia to contract the writers of the works to rewrite that story in a light novel format with an illustrator and editor. After all of that, the companies began to release their media mix efforts with manga adaptations and then anime adaptations for their works.
There's also a reason for why Kadokawa has for years a website called Kakuyomu where people can publish and read web novels, which they created to compete with Narou. And Line also did the same thing and established this year LINE Novel, an app and website where you can publish and read novels, to also compete with Narou. And like with Narou, the most popular works are often picked by the publishers to be light novels. The current anime adaptation "The is Overly Cautious" originally began on Kadokawa's Kakuyomu where the own publisher of course picked it, published as a LN, made a manga adaptation and in this year the anime is out.
Here's the Kakuyomu site:
https://kakuyomu.jp/
Here's Shosetsuka ni Narou:
http://syosetu.com/
Here's a list of works that originally began on Narou (ofc, not everything, only the ones that got anime and others which are included on wikipedia), which includes works like Kusuriya no Horigoto, I want to Eat your Pancreas and Mahouka (there's many Isekai out there, but there's also other works which aren't it)
en.wikipedia.org
And of course, this thing of publishers picking up amateur works on the internet and publishing them isn't an exclusivity of light novels, as the same happens with manga. Many manga on the 2010s began originally on twitter, pixiv or in other parts of the internet before being picked to be officially released like Wotakoi, Tsurezure Children, Shogi Girl (the one from Takagi-san author), Nagatoro-san, Anjou-san, One Punch Man (which had a remake of the webcomic by Murata on Shueisha), Pseudo Harem and so many others.