Board Thread:Skyrim/@comment-109.108.7.9-20150406094840/@comment-187.66.155.114-20180713093304

Every single living being in Tamriel should fear the Greybeards. Because fear is, to an extent, respect. The Greybeards could shake the ground miles away and cause massive avalanches just by whispering, what an all-out shout could possibly do is incalculable.

Yes, they are indeed mortals (very powerful ones) so assassination attemps i.e. while they sleep could possibly slay them down for good, BUT considering how they lived pretty much all their lives under absolute seclusion in High Hrothgar PLUS they have a Dragon up there, I doubt that even the most skilled Thalmor assassins could get there without being noticed, and once noticed they're pretty much done.

Last but definetely not least, if you assume that the Thalmor's ultimate goal is to destroy/deactivate The Towers in order to disrupt Nirn and regain their immortality, they will inevitably have to deal with the Greybeards at some point since they literally live under one of the Towers (the Throat of The World).