Toi: I think there’s a Pashut Chiluk between praising their greatness and saying what we can learn from their good deeds. Also, the Acharonim say that the source is that it’s similar to a Matnas Chinam. This wouldn’t be Stam for no reason. Thanking for an actual service (especially since a lack of gratitude might compel others to not do similar things in a similar situation) should very much be Muttar, I wuold think.
Nisht: There were philosophical Rishonim and Geonim who held that gratitude was the first basic principle on which the world runs.