@InsaneInsomniac
Again i don't wanna make assumptions seeing as this is only the 2nd chapter but there are a lot of reasons why they would no longer have access to magic. But we do have some information that i can use to make assumptions/theories.
1. The MC makes a new body instead of using his own which we can surmise to say that magic cannot stop atrophy so the extension of life via magic doesn't seem possible.
2. Magic can harm the human body with frequent use, another reason as to why he made a new body instead of just staying in his old one.
Those two statements are the reason as to why the old mages died out sooner rather then extending their lives.
It's a bit much to generalize that all of magic over the 200 years is gone after him reading only 1 book but it could be said that the answer is that simply the general population are not good with magic but there are specific mages towers that practice magic to a higher degree that are secluded and only practice/refine magic.
The result of that is that the new mages learnt from the shit books and again the books can be explained as the proper wizards teaching the apprentice > the apprentice writes down what they learnt from it but after handwriting a few books "optimized" the writing process down so that the original textbook is distorted through time because the resulting wizards learning the first textbook learnt it wrongly and advanced and tried to optimize it in the wrong way, TL
R an intergenerational chinese whisper book.
These are all just assumptions and i don't mind you poking at the authors logic but at least give him a chance to explain why/how it happened past chapter 2 and then you can poke at how dumb it is
Which it inevitably will be knowing Japanese authors.