Ok, so I had a good discussion with a Japanese guy about MT and he said that the term "Grandfather of Isekai" is a western term given to MT, though he believes that it's apt.

In Japan, MT is referred to as "Pioneer of Narou type novels", with the type referring here to Isekai novels and Narou being the site the web novel was posted on.

To go a bit deeper into what we talked about, as to why MT and not something else became that popular back then to be called pioneer or grandfather of the modern isekai boom.

Disclaimer: The Japanese guy is in his 40’s and has been following the web novel scene for a long time and he is a big Re:Zero fan and there used to be a whole rivalry with MT fans back in the day. These are his words and you choose whether to believe him or not. Regardless, what he shared was very interesting.

Ok, to start off there are two big novel sites in Japan, Arcadia and Narou, both of whom were competing throughout 2000’s-2010’s for web novel market share. Basically, anyone with a PC and internet can publish their stories on those two sites for free. Narou has titles such as Re:Zero, Slime, MT, Konosuba, etc, whereas Arcadia has titles such as Overlord, Youjo Senki and Gate (these actually came before Re:Zero and MT). As you can guess from the titles, there is a difference between the two sites, Arcadia is more “novel-like” meaning it feels as if a pro is writing it, the stories generally have a much more serious undertone and usually the protagonists are kinda different from usual light novel protagonists. However, stories from Narou are what you tend to associate with Isekai as we know it and Narou is more popular than Arcadia.
Nagatsuki Tappei (ReZero): It goes without saying that "Mushoku Tensei" is interesting, but first of all, it's amazing that you were able to depict the life of one character, Rudy. People say that "CLANNAD" is life, but for me, "Mushoku Tensei" is life

Now, 2012 was probably the most important year of the last decade for anime. Re:Zero started its run in April, MT in September and Konsuba in December, but most importantly, it was the year SAO started airing. The first episode of SAO was released in July, and the first chapter of MT began in September. I think one can figure out the connection easily. SAO was such a hit that people wanted more, and MT was just plain lucky to be released just at the peak of SAO insanity in Japan. However, I did ask him why MT and not Re:Zero. According to him, no one knows. Narou has daily, weekly, and monthly charts just like mangadex. If you have ever seen one of these charts, you would know that its hard for new series to hit the top rankings especially when the story is in its first few months of serialization. Re:Zero was creeping up towards the top slowly whereas MT became an instant hit as soon as it dropped.

Between August and December, people visiting Narou rose exponentially because of SAO, and most of the traffic got driven towards MT as it became the poster boy of Narou. Seeing the booming popularity of isekai, the site started to get flooded with MT-copycats. Hence, the term “Pioneer of of Narou type novels”. So, why MT got the title and not SAO. According to him, while SAO provided a base for gamification of the world, MT provided a much more concrete structure of an isekai world. Before MT, isekai was much more free-flowing and even though the stories could be grouped as isekai like Overlord, Youjo Senki, 12 kingdoms they were also different in their own way. Post-MT, the stories became much more similar in structure and format and the way isekai tropes were used, it was easy to spot where it was coming from. I think it will be much clearer for anime-only’s once the anime goes more deeper into the story.


Now, Konosuba actually has a very interesting part to play in this. It released in December, when MT was at the peak of Turning Point 1 hype. The number of copycats being spawned was increasing day-by-day. In comes Konosuba, as a parody of all the copycats of SAO and MT. It was pretty clear when Konosuba started serializing, that it was poking fun at all the new people trying to jump in on the isekai train, and it was pretty successful.

MT remained no.1 throughout its run, but that came with its own downsides. Unlike Re:Zero which had a mediocre popularity, once it became a light novel and an anime, a lot of people started buying the light novels to catch up. However, since a large chunk of isekai target audience had already consumed MT for free in the WN format on Narou, the LN sales figure were underwhelming compared to its popularity. After all, Narou is a site where people can read for free. Let’s see what happens after the anime, according to him, the response on JP forums has been phenomenal, though there is a good minority that has been criticizing the creepiness over there as well.