Sometimes I forget that other people don't know how computer programing works.
Then I see posts like this one.
Really, I see people asking for new announcer voices, and map-skins, and that is totally doable. The technology is already in place, and it would take a small number of KB sized files to make the announcers work.
But to bring back the old champions and items?
Yeah, they could do it, I suppose.
But I'm pretty sure the code for some of those champions has been completely trashed. It may exist on a backup somewhere, but more than likely, they would have to rebuild those champions and/or their models from scratch. Given the current rework schedule, I'd say bringing back the old champions would take ... well, its easier than testing an origional design, so lets estimate low and say ~~1 years work to bring them all back.
Is it really worth an entire year of labor, or even half that to create a non-core game mode?