Personally, the idea that the series finale has a lead-up is a good one. Stories change all the time and no series ending is able to stand on its own merits without factoring in everything that preceded it. What often makes the final moments so satisfying are the seasons of character development and storylines that came before and Supernatural has had plenty of that over 15 seasons. I imagine the series finale’s final moments will be all the more emotional because of it.