I agree, the afo gave tomura decay theory is so intriguing! there are quite a few things that that would make so much sense in that case: the guy at the door in the manga during tomura's backstory; the fact that his quirk seems to negatively affect his body (which ofc also happens in other cases, see dabi, but it always happens when quirkless ppl get a quirk, see both midoriya and aoyama); the whole tomura being related to nana shimura, etc. And afo is definitely petty enough to search for nana shimura's grandson, and he loves giving and taking quirks so ppl are bound to him, so it's definitely not out of character for him. Also I love parallels and there are already so many parallels between midoriya and tomura, what's one more?
Yes but i don't think Tenko was quirkless.
Above all that the reason that made me believe that theory was the thing about Nana's quirk: skywalk (like skywalker and Tomura would be like luke, but that's other thing aksjaks)
We know Nana's quirk was skywalk and that Izuku has it now because he can float. Right?
So, that means that Nana died with her quirk intact for it to be stored inside the OfA. We've already seen that afo not only can't steal ofa itself but also the original quirks of the holders. (If he could, we would have seen him had at least one)
Having that in mind, then how do you explain that AfO has Skywalking?
He has it, it was mentioned ESPECIFICALLY during his fight with all might and he even says is Nana's quirk before we knew her's was in ofa stored safely.
He stole it, of course. But from who?
It is possible that he stole it from Tenko himself and gave him the decay in exchange without the boy noticing.
Is the only logical explanation.
(There is also this theory about deku being the original decay holder that sounds pretty bananas until...you kinda think about it, and also the theory that afo was in fact kotaro's father but people are not ready for that conversation akdbakdbakd i know it sounds super weird but once you hear it it makes a lot of sense and also would explain Tomura and Yoichi's resemblance)