The ending felt a lot like having their cake and eating it too.
I think the plan was for the Island to be purgatory but when that was sussed in the first season and they denied it they backed themselves into a corner of trying to throw people off the scent.
They shouldn't have claimed that all mysteries would have a scientific explanation either. I was really into LOST theories at the time and some fan fictions I read had better endings than the shit show we got. I'm fairly certain they'd always maintained the smoke monster would be explained in a satisfactory, scientific way. Instead we got Cain and Abel story about how a man was killed by his brother, floated into a light and became a smoky apparition.
Character development was the shows strong point and while I hated some characters (Jack and Kate) and they butchered my boys arc (Syed) overall they did a good job of building up the characters over the 7 seasons.