When you consider the opening warning that it's horrifying and shocking and not for the faint of heart, it's really not surprising that they had to end it on a lighthearted note.

@Phasmophobia said:

I wish it had ended at the burning windmill. Too bad they had to tack on a "happy" ending at the last minute (as many of these old horror films did).

That's certainly a valid opinion but personally I find it hard to make any criticism of this masterpiece of a film.

It was scary to the audience of the time. The exvorist may not have been scary, but audiences were passing out

We got the sequels we did because of that ending. Henry surviving the windmill is essential to Bride of Frankenstein and having descendants carrying on the family business in Son of Frankenstein and Ghost of Frankenstein.

The movie was already pushing many boundaries at the time that were already controversial (man creating man; Playing God), plus this still beautifully lead to the brilliant and what some consider the best Universal Monster movie ever..Bride of Frankenstein!