I both agree and disagree.
I think they should've restarted the series with the current vampire and werewolf characters and actors. I really believe they're both great characters and great actors and if the show is a success from now on, it'll be because of these two and the chemistry they have onscreen.
Honestly, I wasn't that fond of the main characters. As I said before, they were way too whiny. It was all angst and crying and reminded me of a teenage melodrama more than a gritty supernatural TV series exploring the primal nature of human beings.
The new characters, however, are much, MUCH less angsty and melodramatic. In fact, all the melodrama seems entirely centred on the remaining 'original' cast member and she has enough of it to almost be too much to bear already.
The only character I'm really upset about them killing off is Herrick. He was pure awesome.
I'd recommend giving the show a 'fresh' view. Try not to look at it through the lens of the previous seasons and view it as a new show with a similar thread. The new werewolf and vampire characters/actors are worth it, IMO.