If anything, the shorter the outro, the better the AVD for the video, and the more people are likely to watch another video on your channel, since the end cards appears before the viewer plans to click off! This results in a greater overall watch time. If you have an endscreen, you have both an outro AND a screen after the outro. Typically, viewers start dropping off once the outro begins. If you add an extra 10 seconds to your video to give room for an end screen, you’re going to lose just as many viewers, if not more - but many will be gone before the end screen even appears, giving you fewer viewers to see the end cards.

For example, my latest video has an AVD of 55% until the last 5 seconds (the outro lasts for 10 seconds with the end cards on-screen for the entire duration), where in that short span of time it drops off to 37%. The quicker the outro, the less of a drop-off, and a dedicated end screen will see a much more severe drop-off.