Note: To pray in these words express that I am unable to give you a better reward for this. Thus, these words carry praise for the benefactors. (Maariful Hadith)
So if we say thank you no reward is promised.
If we say Jazakallah o khair we will get reward for following a Sunnah.
And as this sunnah is fading if we will revive it we may get Extra award for reviving it.