Made the protagonist being a boring overpowered weirdo with no real personality become the standard for trash isekai

SAO was originally written way back in 2001 for a short story competition. However, because the author went over the word limit, he just decided to publish SAO as a web novel instead. Over the decade after that, the author was just continuing and building off of it. The problem was that the author was self-publishing it, meaning he wasn't really profiting from it.

When it ended up getting published (due to Accel World winning the 2008 short story competition) and got an anime adaptation in 2012, it jumped in popularity and got a ton of money.

SAO didn't really "redefine the isekai genre" or whatever. "Being sent to another world" has been a genre for decades now (i.e. Alice in Wonderland).

What SAO actually did (and why it won the LN of the decade award) was that it encouraged a lot of self-publishing WN authors to shoot their shot. Seeing SAO's author going from someone relatively unknown to becoming the author of an extremely popular franchise gave them hope that they could also achieve such fortune. Additionally, SAO becoming popular encouraged a lot of anime production companies to look more into WNs/new LN series.

It just happened that a lot of these WN authors ended up writing isekai (hence why we have a ton of isekai adaptations every season). Though, a bunch of "non-lazy" isekai and non-isekai WNs have popped up as well.