It’s literally in the description from the first ten chapters as the mechanic it has for passing on power. I’m surprised people did not see it coming.

I wasn't subbed here when were still catching up to the manga and we got 1 chapter every 1-2 days but I was a lurker.

One for All also passing the previous user's quirks was brought up as a possibility multiple times and we weren't even in chapter 20. Just like the existence of a quirk called "All for One" that steals quirks from other users was predicted way before it happened in the manga.

Hell, ever since Deku had that dream at the beginning of the arc people have been calling it in every single chapter thread.

It's like if Dabi ends up being Endeavor's son, it's not written in stone but if you're caught off guard if it does happen to be confirmed then you just weren't paying attention.