You're looking at it the wrong way. People get into their own little Youtube zone jumping from video to video and they need to be guided to do simple things like subscribe and like your videos. It's just a suggestion. It's not as if they had intention of it and you are twisting their arm. They were ready to go on to another video, heard your suggested, said to themselves 'oh yeah, I did kinda like that video...let me go ahead and subscribe'.
- almost everyone in the thread
The problem I have with waiting until the end is, how many of your viewers watch your videos through to the very end? For most people the answer is not a lot. Unless your viewers are all watching until the end, I believe it would be a poor strategy to wait until the end to talk about subscribing. I would consider (and actually, I should do this myself and I think I will) editing a subtle reminder text into the video itself. Something that appears a couple times throughout but is very subtle, like toward the bottom so that it won't detract from the video itself. Many people do not have the attention span to watch entire 15, 20, 25 minute videos. This doesn't mean they don't like the content and wouldn't subscribe if prompted. That's why I believe a hard-edited subtle text reminder during the video might be a good idea (and then still the end slate suggestion). What is the point in asking them at the end if most of them are gone by then?