for manufacturers to open-source old code
this is not going to happen, unless they're forced by law. And even forced by law isn't going to stop them, unless backed by fines. And even fines aren't going to stop them, unless they're BIG fines. And even big fines aren't going to stop them, unless the process to apply those fines is quick. And the "quick" isn't going to happen (I know me intellectual rights and I have the finest lawyers to prove it. Over the next 10 years, in court).
It would be a beautiful world indeed if both processes were tied together, i.e. the moment they stop supporting a handset, they're forced to release the code. In real world, fewer and fewer android handsets can be rooted (by willing owners, not by the 3-letter-agencies and other bad guys)