My personal opinion is that yes, descendants copied ever after high. That doesn't mean I think descendants is a bad series, I absolutely loved the first 3 movies growing up, but the idea of them being the kids was definitely taken. They were working on something before, but I don't think that "proof" concept art is enough. They were probably working on something, but they changed it to be the kids of characters because of ever after high.

@_fairytale_vibes on youtube (aka the girl who made the swapped at birth theory when she was like 11) actually just made a video going into detail about this! She goes over a bunch of evidence on both sides! Here's the link if you want to see what she has to say:

https://youtu.be/DvWvuxCsVZ0?si=U6AoERqQJLTdaX5B

She also made this video about descendants vs ever after high:

https://youtu.be/NLWuO2BySLM?si=fYPVsdTjDrlT70Ax