^Sure, that would be the best hope, of course, and I'd love it if there were some vault that turned out to have every remaining lost episode. I'm just thinking that maybe there's a last-ditch option to be investigated when all else fails, and I'm wondering if anyone involved in Doctor Who restoration has given any thought to tracking down the actual physical tapes and determining if there's a way to extract erased data from them.
There's apparently only less than 20 quad tapes that originated with 405-line recordings on them still in existence at the BBC (or Windmill Road, or wherever the masters are stored these days), so considering if any of those were
actually Doctor Who, the first 187 episodes of the show were shot on 405-line tapes, 81 of which are missing episodes - the most you'd get back would be "less than 20".
But they're not. Of the 625-line tapes only 25 are missing episodes, and only The Enemy of the World 3 has ever been tracked down.
It's likely they're all gone forever...