I really liked the finale. It fits in well with the 15 seasons that preceded it. If you wanted an epic showpiece, I can see why you might be disappointed. But the ending made sense to me on a lot of levels. I think it will work particularly well at the end of a big binge when people go back to watch it years from now.
Dean got the ending he'd always expected, Sam got the ended he'd always wanted. Neither had realised on how empty they'd feel without the other in that scenario, so the writers wanted to explore how that might play out. Maybe a bit melodramatic, but I don't think there was any other way to wrap up a story like this. The show needed to present both brothers as ultimately mortal in order to make the narrative from season 1 episode 1 work retrospectively.
The first death scene was because the brothers had lost their plot armour without Chuck, so I think the anti-climactic nature of it was the right move for a series where the characters had evaded death so improbably so many times.
Episode 19 did the big wrap up if you were more interested in the season long arcs. But I liked the reflective nature of the of episode 20.
After 15 years, it was appropriate for Supernatural to finish out the series on a feeling of loss, vulnerability but still a smidgen of hope. It was like holding up a mirror to the viewer in that respect. Anything more fan-servicey would have a been a cop out, imo.