Dexter is a drama series, no doubt about it, however, what was the reason for letting the main character live in the end? If thinking from moral perspective, Dexter is a killer that "gets away", so what lesson then do we really learn?
Dexter is a survivor. It's something he's always managed, despite his "dark passenger". The end of Dexter shows him doing just that, surviving. Getting on with things.
He's lost Rita and Deb. He won't get Rita's kids; and doesn't really want them anyway. All he has left are Hanna and Harrison. Along the way, he's learned that people close to him hate what it is he does, get killed, or both.
By faking his own death, he knows that Hannah will look after Harrison, and they will both be safe from him and anyone who comes after him either from Florida, or from any future dealings. He doesn't want another Trinity incident with either of them.
He's able to blend in as a no-one doing a menial job where it's unlikely he will be missed for a day or two (unlike his rockstar presence in Miami PD forensics), which means should the urge arise, he'd be able to prepare fully and not need to rush to get back to work.
Arguably, Dexter might not be the killer that "gets away", so much as the one who hasn't been caught yet. Time caught up with all of his victims, some of whom (Trinity specifically) were at it for decades without being caught.
It teaches us that even if we do bad things (ok, Dexter was beyond any normal definition of just 'bad'), that we can put in place measures to protect those around us and try to put the past behind.
Time might yet catch up with Dexter.
Because Showtime wouldn't let him die:
"How about that finale?" said Dexter producer John Goldwyn at the NYFF premiere of The Secret Life of Walter Mitty this weekend, when we asked how the writers decided on that ending. "They won't let us kill him," he told us. "Showtime was very clear about that. When we told them the arc for the last season, they just said, 'Just to be clear, he's going to live.' There were a lot of endings discussed because it was a very interesting problem to solve, to bring it to a close. People have a relationship with Dexter, even if it doesn't have the size and the ferocity of the fan base for Breaking Bad. But it has a very core loyal following."
I suspect they were/are still hoping to revisit the (lucrative) franchise in the future.
Debras passing away, just as everything started looking good for her, getting back together with Quinn etc. combined with Dexter, the really bad guy getting away (despite him loosing everything in the process). Leaves a deep feeling of unfairness. The wrong person died.
Even though Dexter, the main character is inherently bad, the viewer grew to love him, but the ending makes up for that and shows the viewer how wrong it really is with people like him getting away. And how much havoc and sadness they leave behind.