All Might knows he's going to die. He's just glad a piece of him lives on to help Deku.
He took over the world, and got thwarted by 7 generations of One for All. Now he's just bitter.
That is not how it works. I hope the anime explains this better when it gets there, 'cause it's confusing, but:
You know how Tokoyami has a bird head, but his actual quirk is the shadow stand thing? That's because he inherited a shadow power and a bird power from each of his parents, and they fused into one quirk. Todoroki is another example, heat and ice powers merged into a "perfect" fusion of both.
One for All is a special case of that. All for One had the power to manipulate quirks. The brother, the original "One for All", had a useless quirk that could only pass itself on. Since this is unnoticeable, everybody assumed he had no quirk.
A4O takes pity on his "useless brother", and transfers a "strength storage" quirk to him, and by the same mechanism that allows fusions like Tokoyami or Todoroki, the two quirks fused into one, creating One for All.
The original quirk doesn't pass all the quirks of the user, it passes itself on, and nothing more. It's pure chance that A4O transfered that quirk to his brother, and that it ended up fusing into a quirk that matters.
That is true, I also went back and read the chapter where its explained. BUT then, what happens when a person with a quirk gets O4A? Here is what I have in mind:
Gen 1: The first used, the younger brother, had a quirk that can pass on power, and then he gets a quirk that stockpiles power from A4O. Both quirks fusions and forms O4A. He passes it down.
Gen 2: The new user gets O4A, by maybe he also had a similar passive ability, maybe something minor, like having his body get stronger slightly faster than normal, or his body producing a special enzime that makes her muscle more flexible and withstand the power of O4A or superior jumping ability, something that the OG O4A didn't have. Now that passive ability gets mixed with O4A and becomes part of it
Gen 3: Let's say the new user now gets the added benefit of his body getting stronger, so this allows for more power to stockpile without risk to the user, let's say he also had a unremarkable quirk that makes him immune to hipnotism or something like that, I dunno, providing an explanation for that one scene at the school festival. This mixes in with good old O4A and makes it stronger.
And so on, we know All Might (Toshinori) didn't have a quirk when he received O4A, but its plausible he was on a similar situation than the original user. Maybe his quirk is that light doesn't reflects properly off his face and that gives him his american style or something silly like that x).
But talking seriously, there is preset rules to allow this to happen. The examples you mention, Todoroki mostly, shows that quirks are a genetic thing and mixing them its effective. The younger brother shows quirks can be passive and awaken through certain situations, in his case the presence of a second quirk in his body, mixing them both. Deku could realistically, within the rules of the series, awaken new abilities from O4A and it wouldn't be farfetch'd or an asspull since the rules are already there.