To be frank Moshi, those statistics alone tell me that most of your subs are not engaged with your channel.
They probably subscribed to your 10 million view video (congratulations by the way...) and have not watched much since - that's because you haven't given them much to watch though, not because YouTube is "broken".
If you enjoy producing high quality animation videos and posting them on YouTube, then go for it - knock yourself out. Some will be hugely successful, some won't. My advice would be to stop blaming YouTube when your more successful videos fall off a cliff. It's all part of being a video creator on YouTube. The platform wasn't created to help you make a living, and there's not a man with a screw driver working to "fix the problem" when your video has reached the end of it's life.
With regards to the guy (or gal) at YouTube support who told you that there was something wrong; the YouTube algorithm is incredibly complex and in constant change and evolution. I have no doubt that there is always "something wrong" that they are trying to put right. Your video with ten million views is twice the size of my entire channel, but it is an insignificant blip in the YouTubeosphere...Google execs would not shed a tear if it disappeared and was never seen again. They have better things to do than fix the algorithm just so that you do not lose views and subscribers.
Sorry if I sound harsh, but I think you are over analysing your problem and need to step back and see it in perspective.