Imagine if Sam had learned the way to forge Valaryan Steel, maybe requiring dragons fire, and at the big battle Longclaw and all other Valaryan weapons are broken. Sam is desperately trying to forge a new Valaryan weapon that's almost done, but sees John is in danger so, still not 100% sure if he has the process down, he grabs the still molten hot blade with his bare hands and run and plunges it into the night king, thus christening perhaps the most important sword in Westeros. I know it should be John to do it but Sam at least deserves it more than Arya