I saw that a lot of people consider those endings as sad, and I agree but in the end those are still good, the first demon's still waiting for the reincarnation and I agree that's fricking sad but they're gonna meet again and the second one used his power to bring the guy back to life so nothing sad here
I saw that coming and that's what I thought when the guy was brought back, but the reaction he had didn't make him look desparate to be back alive moreover the demon got that pseudo punishment making him honest so ... they're getting a chance to have a better relation I think. To me it looked like the author made this ending quite positive.
As to what the future holds, you're right, being forced alive for a long time is sad, but they could manage that life or not, we don't know.
The fact is their last chapter ends with them being lovey-dovey and the guy acting kinda tsundere so I'll take that to say "nothing sad", but I know the possibility of them being unhappy exist :)
I feel the sadness coming, but the previous two couples had a good ending so I have hope for this last one.