Letang for me is in the same category as guys like Patrick Marleau and Joe Pavelski. At the end of their career, you can look at their overall numbers and squint your eyes and think you see a HOF'er but at no single point during their careers as it was unfolding did I think I was watching a HOF player or one of the games truly elite players. (obviously Letang has cups tho and they don't).
Letang was never seriously considered among the elite with guys like Keith, Weber, Doughty, Chara, Hedman, Karlsson etc and the element he really excelled at (offence), he was usually too injured to have that manifest as high scoring seasons.
If he played full seasons he probably has eight or nine 60-70 point seasons and gets in but he didn't, he was always missing just enough games to prevent that.