No, jailbreaking wont ever be mainstream. It offers very little benefit and a lot of hassle. Its a fringe option that 99% of the public (despite the impression you get on a techie niche like MR) will never have any interest in.
As for piracy, thats the main benefit, and its a shamebut this clarification wont foster piracy because pirates dont care whats legal anyway.
As for malware... sure, maybeadditional security flaws could potentially be introduced by jailbreaking. But thats the least of the problems. People who embark on jailbreaking are prepared to research the issues, keep on top of them as they change, and solve them as they arise.
Apples walled garden is VERY beneficial to users. It makes their products far better and more functional (and is far less walled than people like to claim). However, if people want to strike out on their own and install some other software, I support that. (Just not piracy; not taking the creative work of developers without paying them.)