The point being that Martyr is as strong a "counter" to Assassin as Bodyguard is to any mafia nightkill role, meaning - a very weak, almost negligible one. You might not kill that one person you wanted and it has the potential to set you back, but it's still one less townie toward victory. Besides, the counter being fun to play would be ideal as well; while playing Investigator as it's meant to be played keeps you in the game and rewards you with the ability to point out a liar, Martyr (and Bodyguard) rewards your keen predictive ability with killing yourself.
So that would rather be saying that Martyr is not pointless, but it's a weak counter, it's not fun to play, its original function is so situational that it's actually stronger when used to combo with townie daykill roles (or that one time against SS, even), it's never
worked as intended in countering Assassin, and it could even be fused with Bodyguard to make a more complete role that covers the Martyr's niche in the role meta.
That said, the most immediate counter ideas are those already there, yeah. Daytime Doctor, Assassin Investigator or Assassin Defuser

Will have to think about it.