Probably there would be "signal decay", but one can still make sense of a text when many of the letters are missing. So the decay would have to be more than 50%

Probably there would be "signal decay", but one can still make sense of a text when many of the letters are missing. So the decay would have to be more than 50% for the message to be completely useless. Also, a million years is quite a long time, I doubt there are many media one could use that one could expect to still be at least half readable in one million years time.