You are talking about screwing the show.Duff.
And we aren't talking about screwing the show, we are talking about ending the show and making CW figure out how they would like to continue it.
Plus what happens if they tell them they want a 6th season now, he extends the storylines to two seasons instead of one and then it turns out the show will end with the fifth season because CW went bye-bye?
Its Warners show (and then the CW's), it isn't Kripke's nor is it ours (as much as I love it). If the studio or the production company want the show to continue then having their employee ending it is a supreme act of screwing them over.
If they want it to continue, it is Kripke's literal job to due what they wish. An executive Producer (specifically head writers) job is to do the show in the manner the studio wishes. Its one of the reasons he is paid higher then any other writer, its because its his duties to follow the wishes of the studio.
If Kripke does leave, I'd rather he end the mytharc with Lucifer like you said above and go back to the monsters of the week, even though I think that will feel anticlimactic. I suppose it's the lesser of two evils than trying to draw out the Lucifer arc on their low budget. Dawn O may want the show to continue, but I have a hard time believing she'd pony up more than a few dimes for the budget.
Smallville sure isn't.
Mswood......oh mswoooooood....honey, sweetie pie? Here's a little tidbit on a wishful thinking reality show subject that I have a sneaking suspicion you might enjoy. It's just a hunch, sweetie. I'm not sure why, but I thought you'd appreciate it.