If you read this your instagram post engagement has likely decreased. I experienced the same during the last couple weeks. My theory is that as people only have a limited amount of time on Instagram, the Instagram story feature is taken a portion of this engagement and therefore posts get less likes and comments.
For sure, Instagram stories are increasing the overall usage of Instagram as well. The mere nature of Instagram stories requires a daily routine whereas with the original Instagram a check every 2 or 3 days could be enough. But I assume that this increase in usage is not enough to give both functions its seperate engagement. Some people that have watched posts in the past may have switched completely to Instagram Stories.
That being said, I’s suggest you start investing some more time in Insta Stories!