Well, Simon also wanted to give Graceless a definitive ending. And yet he did come up with a sequel...
As I noted above - Simon is one person. Scott and Alex BOTH wanted to end Dorian. They'd both need to agree to go back on it and since they both seem sure 5 - and a bunch of specials - is as much as they want to do. Surely with two of them saying "I'm done" it's twice as unlikely to come back You could well be right though if we wait long enough. Strange things DO happen. I never though Paul Magrs would work with BF again and let Iris come back.
I would NOT want Dorian recast (more than the older one we've had in-Universe) but I doubt BF would ever be so silly. I'd also not care for anyone but Scott to produce it either and I'm sure Alex wouldn't be that keen on changing the team anyway.
Either way - we've still got 2 series and Christmas to come. Plenty to chew on.
I have it bouncing around my head about them talking about ending it at series 5. And that in regards to ever picking it up, it was "If there is an idea we want to do" type thing. They seemed to know the story they wanted tell, and they don't want to feel pressured to make more unless they have something they want to do.
Or make him a permanent resident of Collinsport, Maine.
I understand your feelings...and would normally share them, as I don't like crossovers. However, Dorian "worked" in his brief crossover to DS, and I am so sad that the range will be coming to a close that I would be willing to accept a crossover to keep him around! However, I know that's not Scott and/or Alex's intent, and anyway, we have no guarantees that DS is continuing either....not yet at least. Although, it seems pretty likely that it will, given that 2016 marks its 50th anniversary.