And if you’re going by prequel “rules,” they (or at least Obi-Wan) believed that Luke - or at least, one of Anakin’s children - was the Chosen One, rather than Anakin himself as they’d all believed before. So, by buying into the prophecy, yeah, he would’ve seen Luke as the “last hope.”
I’m glad that the new movies are completely am bonding the mentions of a “prophecy” and “the chosen one.” It was never mentioned in the original trilogy, so why did Lucas introduce it in the prequels. So rather than killing Palpatine because he loved his son, Vader killed him because the “prophecy” foretold that he would “bring balance to the Force.”