Don't we get the thoughts of characters who've died before they've told anyone what they were thinking? And haven't some of those they might have told also met sticky ends before, say, recording their conversations with ex-PoV characters?But that's like saying one person couldn't write a history of The Wars Of the Roses, or Waterloo. If Sam does become the writer of ASOIAF, then noone's suggesting that he would do it immediately after events finish; more likely he would do it later in his life, as a maester, consulting the records and survivors to tell all those tales (whoever the survivors may be).
It isn't as if PoV characters are invulnerable/immortal in ASoIaF....