It's worth noting that there are three levels here:

1. Preserving the "final copy" - so many old films literally do not exist in any format whatsoever. A DVD or Blu-ray or Laserdisc meets this.

2. Preserving the "best copy" - some things we have in Laserdisc only. It's not the best version ever made, and it's potentially not perfectly preserved.

3. Preserving the "master/source" - this is much more difficult, because these are never released outside the studio, and can be much more than just the "Final Cut" - source files, 3D models, the software used, etc.