I do agree that if Finn had sacrificed himself to destroy the battering-ram laser it would have been an original and surprising plot twist to have one of the main three characters sacrifice themselves like that, and would have added to the Rogue One-ish sacrifice made by the bombers at the beginning. However, it would have meant killing off the most original and most interesting of the main three sequel characters, meaning that we would have to have more of Rey (while I do think she is more proactive and less whiny than Luke, she does have quite a lot of similarities to him) and Poe (who is very much a Captain Kirk kind of character that we’ve seen loads of times before, although that would give an opportunity to develop his character more) in Episode IX. One thing I thought that was quite good was that Rose’s message of ‘protecting what we love’ rather than ‘fighting what we hate’ embodies what Luke ultimately does, and what any Jedi should do - rather than charging on to fight the enemy, lightsaber in hand, like a Sith would do, Luke uses his powers to protect and save his friends and relation in the Resistance by using Kylo’s hatred against him.