Thanks for clarifying: viewing hours may decrease if you delete a video. In addition, this refers only to public viewing hours; videos that are set to private and that are not listed are not counted. A key segment of data to check is the viewing time between subscribers and non-subscribers. The decrease in watch time among subscribers suggests that there's a problem with the direction of your content.
As long as you get more viewing time this year than last year, your viewing time (the last 365 days) will continue to increase. In order to earn revenue on YouTube, you need 1000 subscribers AND reach 4000 hours of playback in the last 365 days. If your youtube channel's watch time has decreased, you can do the following to better understand what's happening. It ensures that YouTubers who can post ads on their videos are active and have a consistent watch time.
Sporadically, YouTube scans its database to identify and eliminate spam activity, fake views, false watch time, and all kinds of data triggered by bots and spammers. Most of the time, many, if not all, YouTubers have data-related issues at the same time. If you delete videos, YouTube will remove the watch time gained on those particular videos.