First of all, if I want to watch season 1 I watch season 1 - not some bizarre reenactment of it. I do appreciate when a finale acknowledges that, you know, things happened between the show's start and finish.
Secondly, the writing is just bad: The pacing is off, the characters are mere caricatures of themselves and the gaping plothole that is Sam not even considering to call an ambualnce while his brother gives a lengthy goodbye speech feeds the suspension of disbelief into a meat grinder.
Also there are sooo many questions left unanswered:
- What happened to the Empty? Jack exploded in there!
- What about Eileen? Why has Sam completely forgotten about his girlfriend?
- How does Dean go from sobbing on the floor / sprinting up the stairs to see Cas to *shrug emoji*?
- Vice versa: How did Cas, who sacrificed himself for Dean, not check on him when he came to heaven?
- How did Jack resurrect him in the first place?
- What happened to the bunker?
- What happened to Miracle?
- Why is the beer in heaven bad?
And finally: The Wig.
So basically, they couldn't have made it worse if they had tried.