Series that are a continuation of the story are different in my opinion. Star Wars and the Lord of the Rings are two that pop into my head.
But the incessant need to put out more numbers just to see how high the average person can count is ridiculous.
Back to the movie series in the original topic, though. (Spoiler alert) Why was it necessary to have the demon kill the girl's family and kidnap the kid in the second one, to find out in the fourth that the demon, the girl, and the girl's mother let him be adopted by another family? WTF? What kind of sense does it make to go through all that to get the kid, just to give the kid away?
It's like the Alien movies. The face huggers couldn't break out of a bell jar with thin glass like they were in in the second movie but they could break through one inch glass of a cryo-chamber in the third movie? What kind of sense is that?